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Efficacy of the Treatment of Developmental Language Disorder: A Systematic Review

Sara rinaldi, maria cristina caselli, valentina cofelice, simonetta d’amico, anna giulia de cagno, giuseppina della corte, maria valeria di martino, brigida di costanzo, maria chiara levorato, roberta penge, tiziana rossetto, alessandra sansavini, simona vecchi, pierluigi zoccolotti.

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Correspondence: [email protected] ; Tel.: +39-0498217670

The authors are representative of the scientific association CLASTA—Communication & Language Acquisition Studies in Typical & Atypical Population.

Received 2021 Jan 15; Accepted 2021 Mar 16; Collection date 2021 Mar.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/ ).

Background. Language disorder is the most frequent developmental disorder in childhood and it has a significant negative impact on children’s development. The goal of the present review was to systematically analyze the effectiveness of interventions in children with developmental language disorder (DLD) from an evidence-based perspective. Methods. We considered systematic reviews, meta-analyses of randomized controlled trials (RCTs), control group cohort studies on any type of intervention aimed at improving children’s skills in the phono-articulatory, phonological, semantic-lexical, and morpho-syntactic fields in preschool and primary school children (up to eight years of age) that were diagnosed with DLD. We identified 27 full-length studies, 26 RCT and one review. Results. Early intensive intervention in three- and four-year-old children has a positive effect on phonological expressive and receptive skills and acquisitions are maintained in the medium term. Less evidence is available on the treatment of expressive vocabulary (and no evidence on receptive vocabulary). Intervention on morphological and syntactic skills has effective results on expressive (but not receptive) skills; however, a number of inconsistent results have also been reported. Only one study reports a positive effect of treatment on inferential narrative skills. Limited evidence is also available on the treatment of meta-phonological skills. More studies investigated the effectiveness of interventions on general language skills, which now appears as a promising area of investigation, even though results are not all consistent. Conclusions. The effectiveness of interventions over expressive and receptive phonological skills, morpho-syntactic skills, as well as inferential skills in narrative context underscores the importance that these trainings be implemented in children with DLD.

Keywords: developmental language disorder, intervention, evidence-based

1. Introduction: Developmental Language Disorder

Language disorder is the most frequent developmental disorder in childhood [ 1 ]; however, it does not constitute a diagnostic category that refers to a homogeneous condition [ 2 , 3 ]. In some cases, the disorder is limited to production; however, in the most serious cases, it extends to the understanding of language. It can also affect different aspects of language processing, such as: (a) the form of language (phonetic, phonological, morphological, morpho-syntactic, and syntactic processing); (b) its content (semantic-lexical and phrasal processing); and (c) its use (pragmatic and discursive processing) [ 4 , 5 , 6 ]. Approximately 11–18% of children aged between 18 and 36 months [ 7 , 8 , 9 ] present a delay in the appearance of expressive language that, in the most severe cases, can also be observed in the receptive domain [ 10 , 11 , 12 ] in the absence of deafness, intellectual disability, brain injury, and cognitive disorder.

These children have been called late talkers [ 13 , 14 ]. The prognosis is generally good, as, in 70% of cases, expressive language improves significantly by three years of age and subsequently the development of language skills is generally in line with the expected performance in typical development [ 9 , 13 , 15 , 16 , 17 , 18 ]. However, some mild difficulties in daily communication interactions may persist [ 19 , 20 ]. Recovering children have been referred to as “late bloomers”. Thus, being a late bloomer does not necessarily imply a negative evolution; evidence indicates that the outcome is likely to be more favorable if the ability to understand language is preserved and there is no history of language and reading problems in the family [ 21 , 22 ]. However, even though many late speakers reach the same level of linguistic development as their peers, in 5–7% of the population the disorder persists after the age of three and a spontaneous recovery of language skills before school age is unlikely. In these cases, we speak of developmental language disorder (DLD) [ 4 , 6 ].

DLD has been defined as a neurodevelopmental disorder that includes a set of variegated clinical pictures that are characterized by delay or disorder in one or more areas of language development in the absence of cognitive, sensory, motor, affective, and important socio-environmental deficiencies [ 3 , 23 ]. The term DLD (e.g., Ref. [ 24 ]) or, more simply, Language Disorder, is now more commonly used instead of the more traditional “Specific Language Impairment” (SLI) [ 25 , 26 , 27 ], because it has been questioned whether language disorder is truly “specific” [ 28 , 29 ]. Indeed, it is well-known that a language disorder is frequently associated with various types of cognitive difficulties, which manifest in different ways, such as, for example, in procedural memory management [ 30 ], motor control [ 31 ], phonological working memory [ 32 ], and executive functioning [ 33 ]. Recently, the CATALISE Consensus [ 34 ] has chosen to use the term “developmental language disorder”, implying that it emerges in the course of development, rather than being acquired or associated with known biomedical causes. Although the term DLD is now frequently used [ 34 ], the terms ‘Primary Language Impairment’ and ‘Primary Language Disorder’ have also been used to account for the a-specificity of this language disorder and its unknown origin [ 24 , 35 , 36 ] (for further details, see the Method section). Thus, following the more recent international consensus [ 34 ], we will refer to language problems throughout the present systematic review in terms of DLD, regardless of how authors of previous papers, as reported in this review, named it.

The language difficulties of children with DLD often have severe consequences in pre-school and early primary school. In approximately 40 to 50% of cases, linguistic impairments lead to negative neuropsychological sequelae [ 9 ], particularly at the time of the change in expressivity from oral to written language, i.e., in the first two years of primary school, when literacy rests on the mapping of the phonetic system [ 37 , 38 ]. It has been shown that language disorder is associated with a high risk of school learning problems [ 39 , 40 ] (estimated as five times higher than in the general population [ 41 , 42 ]), behavioral and psychiatric problems [ 43 , 44 ], and disturbances in emotional and social adaptation [ 45 , 46 ]. Additionally, there is evidence that these problems persist in adulthood and throughout a person’s life, also affecting job opportunities [ 47 , 48 , 49 ].

The breadth of these problems and their negative impact on a child’s development indicates the importance of an early identification of children who risk of exhibiting DLD or related problems, with the potential benefit of promoting interventions in an age group in which significant improvement is most likely to occur [ 50 ]. Effective and early diagnosis can also facilitate the planning of targeted rehabilitation interventions before problems interfere with the formal education process [ 6 , 51 ].

Interventions for the Developmental Language Disorder

Therefore, identifying effective interventions is a fundamental aim in clinical practice with children who have DLD. In fact, language intervention during development may not only have short-term outcomes on the language component treated, but also medium- and long-term influences on the global development path. The links between oral language acquisition and written language learning are well-known; also important are the negative consequences on the quality of social integration and the emotional development of children with language disorder (e.g., Ref. [ 44 ]). At the same time, research in this area is made complex by the need to identify modalities of intervention that, on the one hand, reflect the variability of the disorder in its components and in different age groups and, on the other hand, take the variables that intervene in determining stable and lasting changes in children with DLD into account.

The international literature offers a wide range of rehabilitation interventions aimed at children with DLD. They not only reflect the wide variability in the expression of language disorder at different ages, but indicate the importance of establishing which rehabilitation intervention provides the best care for children with DLD. The goal of the present study was to systematically analyze the effectiveness of interventions on children with DLD from an evidence-based perspective. This effort is not new. In particular, Law et al. [ 52 ] carried out a systematic review of the RCT studies on the effectiveness of language intervention. They found clearer results in the case of expressive phonological and vocabulary difficulties than in the case of receptive difficulties. Evidence on expressive syntax interventions was mixed, which indicated the need for further research. In fact, considerable research has been carried out since this review. Other reviews have focused on more particular issues, such as specific areas of language intervention (i.e., narrative-based interventions [ 53 , 54 ] and phonological and associated expressive language difficulties [ 55 ]), the mediating role of short-term memory over the efficacy of language intervention [ 56 ], or the use of videos and digital media in interventions that were carried out by parents [ 57 ]. As a considerable amount on new studies have been made available since the last systematic general review [ 52 ], it also seemed to be important to carry out an updated review of the literature. We also felt that this was timely since the described shift in perspective leading to an interpretation of language difficulties in terms of DLD (e.g., Ref. [ 24 ]).

Note that we focused on interventions on different aspects of language (e.g., phonetics/phonology; vocabulary; and, morphology/syntax) and domains (comprehension and production) and did not analyze studies that were concerned with the improvement of pragmatic skills. Most of the interventions deal with language production, but a few investigate interventions aimed at reception/comprehension. Some of the approaches in the literature adopted techniques and protocols that aimed at single components (minimal phonological pairs or particular morphological and syntactic deficit characteristics), while other approaches aimed at a wider and “ecological” stimulation of different aspects of language production.

The methods of administration depend, in part, on the approach, and can involve the intervention of specifically trained figures (speech and language therapists), educators, and teachers, or, increasingly, interventions mediated by parents with different training and supervision by clinicians. Another important aspect in evaluating the effectiveness of the interventions is the way the outcomes are evaluated. In fact, the tools that are used for diagnosis are often not very sensitive to change and the tools built ad hoc often only measure the skill being trained and do not allow for evaluating generalization to neighboring skills or other language domains.

The location of the intervention also varies according to the different approaches. In addition to the interventions that were carried out in the clinical setting, many interventions are carried out at school or at home (for a review, see [ 58 ]).

The frequency and duration of the interventions also appear to be very variable. Generally, individual interventions are tested in short and relatively low-intensity cycles, on very specific targets, and with the frequent absence of follow-up evaluations. Group interventions are more rarely described (both groups of children with DLD and the child with DLD within a typical developmental peer group).

Another element of complexity for the “evidence-based” identification of the effectiveness of treatments is the variability that is linked to the characteristics of the language. Therefore, it is necessary to carefully consider the applicability of the treatments (often developed in an Anglo-Saxon context) to other languages and the possible influences of the characteristics of our language on the effectiveness of the treatment itself.

Below, we report a systematic review (SR) of the studies on the effectiveness of intervention on children with DLD (including previous SRs, RCTs, and cohort studies).

A review of the literature was carried out as part of a Consensus Conference about the diagnosis and treatment of children with language disorder, which was held in Italy in November 2018 [ 36 ]. This Consensus Conference agreed to use the term ‘Primary Language Disorder’ for the Italian context, with it being clearer to clinicians in defining its origin not acquired or associated with a known biomedical cause, as detailed in the first paragraph of the Introduction. For this reason, the meaning of the term ‘Primary Language Disorder’, which was used in this Consensus Conference, corresponds to that of the term DLD, as recently adopted [ 34 ] and used throughout the present systematic review. The organization and implementation of this Consensus Conference followed the steps that were indicated in the Methodological Manual of the Italian Superior Institute of Health [ 59 ]. The review was prepared according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [ 60 ], and it includes a PRISMA flow diagram.

2.1. Selection Criteria

The clinical question was formulated while using the PICO approach and the criteria for inclusion and exclusion of studies were established a priori (see Table 1 ).

Inclusion and exclusion criteria.

2.2. Source of Data and Screening

A systematic search of the literature published up until December 2020 was conducted through research on the following databases PubMed, Embase, Web of Science, The Cochrane Central Register of Controlled Trials (CENTRAL; 2020 Issue 11), SpeechBITE (speechbite.com; accessed on 30 December 2020), and PsycINFO (Ovid). In addition, we searched the clinical trials registers: ClinicalTrials.gov (clinicaltrials.gov accessed on 30 December 2020), World Health Organization International, and Clinical Trials Registry Platform (WHO ICTRP; who.int/trialsearch; accessed on 30 December 2020) for ongoing or unpublished trials on December 2020.

For each database a search strategy was developed by considering MESH terms and free terms (see Appendix A ). In addition, further articles were identified by screening the reference lists of relevant reviews. Finally, experts and practitioners in the field, participants in the scientific technical committee, or working groups of the Consensus Conference indicated further potentially relevant studies.

2.3. Data Selection, Extraction and Quality Assessment

Titles, abstracts, and full text screening were performed by two independent reviewers. Disagreements after a full text review were resolved through discussion. Three independent reviewers extracted data from each included study. Information was extracted concerning: study design, population characteristics, type of test or treatment, type of comparison group, results, type of setting and figures involved, and the results of the studies. We did not contact the authors of relevant studies reporting incomplete data to request the missing information.

An evaluation of the quality and usability of the results of the reviewed studies was carried out by three independent reviewers.

The checklist “AMSTAR 2” [ 61 ] was used to evaluate the internal validity of systematic reviews. AMSTAR 2 is composed of 11 items: (1) “a priori” design; (2) duplicate study selection and data extraction; (3) comprehensive literature search; (4) the status of publication as an inclusion criterion; (5) list of studies; (6) characteristics of the included studies; (7) assessment of the scientific quality of the included studies; (8) use of the scientific quality in formulating the conclusions; (9) methods used to combine the findings of studies; (10) likelihood of publication bias; and, 11) conflict of interest).

For each criterion, the ‘yes’ (clearly done), ‘no’ (clearly not done), or ‘not clear or not applicable’ category was assigned. The revisions were then classified, as follows:

from 8 to 12 criteria with the a “yes” assessment: high quality;

from 4 to 7 criteria with the “yes” assessment: medium quality; and,

3 or less criteria with the “yes” assessment: low quality.

For RCT studies, based on the criteria that were developed by the Cochrane Collaboration [ 62 ], the following dimensions were assessed:

random sequence generation (selection bias), which considers the risk that the allocation of subjects in the experimental and control groups may have occurred in a non-random way, indicating a possible problem in the selection of groups;

allocation concealment (selection bias), which evaluates the degree of protection against the risk that the trial operators are aware of the mechanism of random allocation of subjects;

the blinding of participants and personnel (performance bias), which considers the risk that the lack of blindness of trial objectives in participants and staff might alter performance (e.g., favoring a lack of expectations for the control group), thus affecting the trial outcome;

blinding of outcome assessment (detection bias), which indicates the risk that persons evaluating the study’s outcome are aware of the group assignment to different forms of intervention, thus influencing the probability of capturing the effects of the intervention;

incomplete outcome data (attrition bias), which evaluates the possibility that the presence of missing data modifies the estimation of the effects of interventions. The reasons for attrition or exclusion were reported as well as whether missing data were balanced across groups or related to outcomes; and,

selective reporting (reporting bias), which indicates the possible risk that only a selection of variables is presented in the report, e.g., the tendency not presenting measures for which the results were not significant (e.g., not presenting measures for insignificant results).

For each of the studies reviewed, an assessment of the possible presence of these bias risks is carried out. Figure 1 and Figure 2 summarize the results of the evaluation.

Risk of bias found in individual studies considered in the present review. Green: low bias risk; yellow: unclear bias risk; red: high bias risk [ 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 ].

Percentages of risk of bias found in the studies considered in the present review.

It is important to take the validity of the knowledge into account in order to assess the generalizability of the results of the included studies, i.e., the possible presence of a bias in the data, as well as transferability to clinical practice (external validity).

External validity was evaluated based on the transferability of results to clinical practice. For each of the RCT studies examined, the setting in which the study was carried out as well as the language covered by the intervention were assessed and are reported in Appendix B .

2.4. Data Synthesis

The characteristics of the included studies are presented in tables and summarized narratively. A meta-analysis of outcomes was not appropriate due to the heterogeneity of the data; however, narrative results are presented.

Using the bibliographic research, we identified 3334 reports after removing duplicates; two independent reviewers excluded 3219 reports on the basis of title and abstract. Any doubtful cases were resolved by discussion with a second reviewer.

We acquired 118 potentially relevant studies in full text, and we assessed their compliance with the a priori defined inclusion criteria. We identified 27 relevant studies [ 52 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 ] and excluded 91, which we considered to be ineligible. Figure 3 shows the PRISMA flow diagram for the selection of the studies.

Prisma flow chart of included studies.

3.1. Characteristics of Studies

Appendix B presents detailed tables with the characteristics of the included studies. Of the included studies, one was an SR [ 52 ] and 26 RCTs [ 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 , 87 , 88 ] (including two studies [ 86 , 87 ] derived from Loo et al.’s [ 89 ] review) on the effectiveness of interventions for the treatment of identified DLDs.

Law et al. [ 52 ] (score AMSTAR = 8) included 36 RCTs that evaluated different types of interventions that aimed at improving one or more of the following areas of language: expressive or receptive phonology, expressive or receptive vocabulary, and expressive or receptive syntax. In particular, the following comparisons were evaluated:

interventions compared to no treatment or later treatments;

specific interventions with respect to general stimulation conditions (e.g., studies in which children in the control group were assigned to conditions that were designed to simulate interaction in therapy without promoting the language area of interest. These are cognitive therapy, general play sessions or speech therapy that did not focus on the area of the specific linguistic deficit considered); and,

interventions compared to other language therapy approaches (e.g., studies comparing what they considered a “traditional treatment” with what they considered to be an experimental treatment. The latter could be a different approach performed by the same person, such as “targeting early” against “late developing sounds”, or the same approach performed by different people, as in the case of “focused stimulation” provided by clinicians against that implemented by parents).

Of the 26 RCTs, six studies evaluated either expressive [ 63 , 64 , 65 , 66 , 67 ] or receptive [ 68 ] phonological skills, one on expressive vocabulary [ 69 ], eight grammar/morphological skills [ 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 ], with one reporting data on both phonological and morpho-syntactic skills [ 78 ], two on narrative skills [ 79 , 80 ]; two examined meta-phonological skills [ 81 , 82 ] and six language skills in general [ 83 , 84 , 85 , 86 , 87 , 88 ], including two [ 87 , 88 ] comparing the effectiveness of the “Fast ForWord Language” (FFW-L) training program with other intervention programs.

3.1.1. Risk of Bias of Included Studies

With regard to the internal validity of RCTs, Figure 2 summarizes the data for the entire sample of 26 studies. Overall, the body of evidence was affected by a definitely high risk of bias in selection bias, since most of the studies did not report the method of randomization. Moreover, we judged most studies at unclear risk of bias of detection bias due to unclear blindness of participants and staff (24 out of 26 studies) and selectivity in the publication of results (19/26 studies).

3.1.2. External Validity

Extrapolated data regarding the language targeted by the intervention and the context of treatment can be seen in Appendix B . Almost all of the included RCTs (21 out of 26) were conducted on the samples of English-speaking children and, in 16 studies, treatment took place in the school setting.

3.2. Effect of Intervention

Below are the studies identified according to the language area (outcome) being treated.

3.2.1. Expressive Phonological Skills

Information regarding the effectiveness of interventions on phonological expressive skills in children with DLD comes from studies that were reviewed in the revision of Law et al. [ 52 ]. In particular, four studies specifically evaluated the effectiveness of several interventions on phonological expressive competence in children with DLD and reported an improvement of outcomes in the target children with DLD with respect to the control groups with no treatment ( N = 264; SMD = 0.44, 95% CI: 0.01, 0.86) [ 50 ]. The effect was larger when treatments given by parents were excluded ( N = 214; SMD = 0.67, 95%CI: 0.19, 1.16). Furthermore, the estimate was larger when only treatment lasting at least eight weeks was considered ( N = 213; SMD = 0.74, 95%CI: 0.14, 1.33).

Five studies investigated the effectiveness of interventions on phonological expressive skills in children with DLD. A brief description of these studies follows (additional characteristics are reported in Appendix B ).

Allen’s study [ 63 ] examined the effectiveness of two interventions, based on the use of maximally contrasting phoneme pairs, performed once or three times a week in children diagnosed with a speech sound disorder (SSD). Children were randomly assigned to one of three groups: one-time-per-week phonological intervention, three-times-per-week phonological intervention, and active control intervention, which was given a single-weekly treatment based on book narration. Based on the score for the percentage of correct consonants (PCC), children in the three-times per week group outperformed the single-weekly intervention group (and the control group) after eight weeks as well as after 24 weeks of training (when the overall training dosage was comparable). At a six-week follow-up, both of the experimental groups showed continued improvement without significant differences between them. Notably, the study did not consider language skills other than phonological ones.

In Lousada et al.’s [ 64 ] study, the effectiveness of a phonological therapy based on the combination of expressive phonological tasks, phonological awareness, and auditory discrimination and listening activities was compared to an articulation therapy that consisted of a traditional approach according to the “Van Riper Method”. At the end of the intervention, both groups showed improvements in verbal production, but children that were assigned to phonological therapy showed a greater improvement in the PCC score and a greater generalization of untreated words as compared to the other group.

In the study conducted by Diaz-Williams [ 65 ], the effectiveness of an intervention called “Gross Motor Activity” was examined: it is characterized by the production of target phonemes in single words, as depicted in four images in association with a motor activity (e.g., jumping). The group that was assigned to this intervention was compared with two other groups that received the “Structured Table Activities” and “Structured Table with Letter-Tracing Activities” interventions, respectively. In the two trainings, the same images were presented; however, in the first one, the words were simply incorporated in the table activities, whereas, in the second one, the children also received a card with the target sound in order to trace the target sound with their finger. All of the children showed a reduction in the average number of phonological errors on the HAPP-3 test [ 90 ] with no significant differences among trainings; moreover, in all cases, the intervention had a positive effect on the children’s homework.

In the study conducted by Wren and Roulstone [ 66 ], the effectiveness of a computer-supported therapy on phonological skills was evaluated. A group of children was presented with experimental software that mirrored board activities using interactive games and was compared with two groups: one group underwent desk therapy, which included a variety of games with images and objects and the other group received no treatment. The results showed no significant differences between the two groups in phonological production (measured with GFTA Sounds in Words subtest [ 91 ] and in terms of PCC), which was also confirmed in a follow-up three weeks after the end of the experiment.

Jesus et al., 2019 [ 67 ] evaluated the effectiveness of a 12-week novel tablet-based approach to phonological intervention targeting children with phonologically based speech sound disorders. A group of children ( N = 22) was assigned to a combination of phonological awareness activities, phonological awareness program, auditory bombardment, and discrimination and listening tasks delivered with a tabletop or with an app running on a tablet. The results showed that both tabletop and tablet-based methods of delivery of a phonological intervention were effective in improving the speech of children. There was a significant improvement in PCC and in the percentage of phonemes correct from baseline (T1) to intervention (T3) for both groups, which was greater during the intervention period (between T2 and T3). Similar results were obtained for the percentage of correct vowel scores, with an improvement being noted at both baseline and intervention, but the increase after intervention was only significantly greater in the tablet group.

Overall, there is evidence that interventions aimed at expressive phonological skills produce appreciable results, even if it is not possible to specify which type of intervention is the most successful.

3.2.2. Receptive Phonological Skills

Law et al. [ 52 ] showed no evidence of the effectiveness of interventions on phonological receptive skills. Only one study was identified that did not present significant differences between the groups.

We found one study that examined the effect of treatment over receptive phonological skills. Roden et al. [ 68 ] investigated whether the Auditory Stimulation Training with Musical material (ASTM) influenced auditory working memory, language processing, phoneme discrimination, and high frequency hearing abilities in preschool children with DPL (with low percentile ranges in the TROG-D [ 92 ]). Children in the experimental group heard acoustically modified music over earphones in small groups of 5–6 children. They revealed significant increases of working memory capacity measures, phonemic discrimination and speech perception at high frequency, and they outperformed control groups (pedagogical activities group, and no intervention groups) in all measurers.

Overall, the evidence on the effect of intervention on receptive phonological skills is too limited to draw any conclusion. Yet, the positive effects that were reported by Roden et al. [ 68 ] suggest the importance that further research will examine this linguistic area.

3.2.3. Expressive Vocabulary

Law et al.’s review [ 52 ] reported the effectiveness of interventions that aimed at improving expressive vocabulary in children with expressive difficulties only when compared with no-intervention ( N = 82; SMD = 1.08, 95% CI: 0.61, 1.55), but not when compared with other cognitive therapies ( N = 25; SMD = 0.62, 95% CI: −0.24, 1.49). A large intervention effect was also present when parental reports of vocabulary were used as dependent measures ( N = 136; SMD = 0.89, 95% CI: 0.21, 1.56).

A cross-over RCT study [ 69 ] evaluated the effectiveness of using e-books as a tool to support vocabulary acquisition in two experiments. The first experiment assessed whether the group of Dutch children with DLD ( N = 29) was able to learn new words through reading electronic storybooks without the support of adults and whether storybooks with video and audio effects were more or less advantageous when compared to electronic versions with static illustrations (i.e., without effects); two stories that children had not heard during the intervention acted as a non-treatment control. The second experiment ( N = 23) had a dual purpose, i.e., to confirm the results of the previous experiment and extend knowledge regarding learning new words in children with DLD by exploring two potential variables, i.e., phonological working memory and language skills. In the first experiment, better performance was obtained with “static” stories; this finding was also confirmed by the second experiment. Children with more severe DLD obtained less of an advantage from e-books when music and sounds were present (probably because they had difficulty in perceiving speech in noisy conditions).

Overall, there is still limited evidence that targeted interventions on expressive vocabulary acquisition produce effective results. It seems important that these partial results be confirmed in future RCT investigations.

3.2.4. Receptive Vocabulary

No studies were identified that investigated the effectiveness of receptive vocabulary interventions in children with DLD.

3.2.5. Morphological and Syntactic Expressive Skills

Law et al.’s [ 52 ] review reported seven studies that examined the effectiveness of different interventions on morphological and syntactic expression skills in children with DLD. Interventions proved to be effective when compared to non-interventions or other cognitive therapies, but the effect was only clear when children with severe language comprehension difficulties ( N = 233; SMD = 1.02, 95%CI: 0.04, 2.01) were excluded.

Eight studies investigated the effectiveness of interventions on morphological and syntactic expressive skills in children with DLD. A brief description of these studies follows (also, see Appendix B ).

The study by Plante et al. [ 70 ] aimed to evaluate the effectiveness of a treatment that used conversational recast for the correction of morphological and grammatical errors that were specific to the English language (such as past -ed, auxiliary -is, third person -s, possessive -s). A group of nine monolingual American children individually received the experimental intervention in the high variability condition (consisting of listening to the morpheme being treated in 24 verbs during each treatment session); a second group of nine children served as a control group and received the intervention in the low variability condition (based on listening to the target morpheme in 12 verbs, each restructured twice in each session). Children in the high variability treatment condition had better results and showed significantly better treatment effects for the target morphemes than the control group. However, the high variability condition produced a significant change in the use of trained, but not untrained, morphemes.

The study by Fey et al. [ 71 ] assessed the effectiveness of an intervention based on the Competing Sources of Input (CSI) hypothesis, which states that, at a certain stage of development, children cannot grasp the difference between subject-verb structures (SV) that appear in isolation and SVs that are part of a larger phrasal construction. In children with DLD, a delay in acquiring this grammatical rule is expected. The intervention, in English, was related to the development of verb morphology and concordance. Three treatment sections were provided for each target morpheme (the auxiliary “is” and the suffix of the third person singular/3S); past tense “-ed” was only monitored as a control. A control group of 11 children carried out a standard stimulation intervention (in which the comprehension activities were focused on semantic contrasts and the production activities included both declarative and interrogative stimuli) of equal duration and frequency. Both of the treatments were carried out, through individual sessions, by a researcher. The children assigned to the CSI group acquired greater skills in the use of “is”-“is” and in the understanding of “is-no”, as compared to the control group. Moreover, for the CSI group, a significant correlation between the understanding of “is-no” and the production of the auxiliary was observed. The difference between the two groups occurred, although the exposure to “is” was the same during the sessions; the authors found a strong support for the CSI. On the other hand, no significant differences emerged between the groups, either in the production of 3S or in the control -ed.

The study conducted by Smith-Lock et al. [ 72 ] evaluated the effectiveness of a grammatical intervention in which children in the experimental group received a cueing strategy, which, after an error, provides a hierarchy of facilitations that aimed at obtaining the correct response. The control group received an intervention characterized by a recasting reformulation strategy, in which, at the same time as the error, the correct target was given to the child without stimulating him/her to produce it. Both of the groups showed improvements in a series of tests [ 93 ] that examined grammar skills (use of pronouns he/she, past -ed, and possessives), but the effect was more evident in the “cueing” group. In the individual analysis, 50% of the children in the cueing group and 12% in the recasting group showed a significant effect of treatment. Finally, in an eight-week follow-up, there was no significant difference between the groups: in each group, half of the children who showed a significant gain in treatment retained it after eight weeks.

In the study by Washington et al. [ 73 , 74 ], the effectiveness of a computer program to improve morpho-syntactic expressive abilities (syntactic order of elements and morphological elements, such as the article “the”, the use of “-ing”, and the auxiliary “is”), was evaluated. In a first study [ 66 ], the experimental group was submitted to a computer-assisted (C-AT) program, called “My Sentence Builder”, which contained images aimed at facilitating the production of sentences and it was compared with two other groups: one group (nC-AT) was given desk activities with predetermined materials and a control group (NT) was given no treatment. The results showed that both of the interventions resulted in improvements in both the morpho-syntactic expressive skills (as assessed by the SPELT-P [ 94 ] tests) and spontaneous use of language when compared with no treatment (as assessed by the DSS [ 95 ] system). No significant differences were observed between the C-AT group and the nC-AT group. At the three-month follow-up, the treated groups showed better performance in grammatical competence as compared to the control group, while no difference was found between the two.

In a follow-up analysis of the same study, Washington et al. [ 74 ] examined the session-to-session progress in terms of efficiency (i.e., the first session in which the child achieved an 80% criterion) and syntactic growth (the individual advancement beyond basic sentence level) in the two groups of children who received the two forms of intervention. The Computer-Assisted Intervention group outperformed the Table-Top Intervention group for efficiency and syntactic growth.

The study by Yoder et al. [ 75 ] assessed whether the pre-treatment mean length of utterance (MLU) was able to predict which intervention, between the “Milieu Language Teaching” (MLT) and the “Broad Target Recasts” (BTR), was more valid in fostering the grammatical development of monolingual English children (aged between 30 and 60 months). MLT and BTR are both treatments that start from child-centered play and use recasts as a consequence of child utterances. MLT focuses on preselected grammatical targets, whereas BTR aims at any developmentally progressive grammatical structure on the basis of the actual utterances. Children who started the treatment with an MLU of ≤1.84 morphemes showed faster grammatical development if they underwent the MLT rather than the BTR treatment. No differences were found between groups in children with initial MLU >1.84 morphemes. Finally, most of the participants maintained grammatical growth after treatment. In fact, as a group, they showed a moderate gain in grammatical development between post-treatment assessment and a follow-up four months after the end of therapy.

Finestack and Fey’s study [ 76 ] compared the effectiveness of deductive and inductive techniques for learning new grammar skills. The deductive instruction, which was carried out with computer support, included a teaching session of the new grammatical morphology through modelling and an explicit auditory suggestion (“prompt”). In the following session, the researcher requested the production of the new grammatical morpheme with images and an explicit auditory prompt, followed by tests to evaluate the learning, generalization, and maintenance of the target morphemes. The inductive instruction provided the same intervention with the difference that the auditory prompt in the recast modelling and restructuring activities was implicit. The explicit approach to teaching new grammatical rules proved to be better than the implicit one. However, several limitations were found, as responses varied considerably between participants, rehearsal contexts, and sessions.

In a related study, Finestack [ 77 ] compared the effect of explicit instructions (aimed to make the learner aware of a given linguistic pattern) with a more traditional implicit approach in children with DLD. In particular, the acquisition, maintenance, and generalization of three novel grammatical forms (gender, aspect, and person targets) was examined after either a training with implicit instruction or a combined explicit-implicit (E-I) instructional approach. The results showed a greater proportion of pattern users (participants with a performance greater than or equal to 80% on a given probe) in the E-I group for the acquisition, maintenance, and generalization of the grammatical forms. The effect was clear when the data on grammatical forms were collapsed together and in the case of the gender target. These findings are in keeping with the idea of a greater effectiveness of interventions incorporating the use of explicit instructions to teach grammatical forms to children with DLD.

Only one RCT assessed the effectiveness of a combined morpho-syntactic and phonological intervention in children with both phonological and morpho-syntactic deficits. This was Tyler et al.’s study [ 78 ], in which two groups were compared to analyze the effects of such interventions on the non-target domain as well as possible variations in efficacy according to the sequence of interventions. The morpho-syntax group showed a significant improvement in both morpho-syntactic and phonological skills as compared to the control group, whereas the phonology group showed a significant improvement in phonological, but not morpho-syntactic skills when compared to the control group. No significant differences emerged in phonological and morpho-syntactic performance between the two treated groups. For both intervention sequences, greater changes were highlighted in phonological than in morpho-syntactic skills, but they were only of significant magnitude in the first group. Each type of intervention led to improvements in the treated domain, but the morpho-syntactic intervention also led to a change in phonological skills that are similar to that obtained by the first phonological intervention. Moreover, the sequence with morpho-syntactic, rather than phonological, treatment also resulted in slightly better overall morphosyntactic performance.

Overall, there is some evidence that interventions aimed at morphological and syntactic expressive skills in children with DLD produce effective results. However, a number of inconsistent results have also been reported, and it is not clear which factors drive these differential outcomes. Furthermore, all of the studies were conducted in English. Given the profound differences in morphological structure between English and other Indo-European languages (such as French, Spanish, and Italian, the main objective of the Consensus Conference from which the present review originates), it appears to be necessary that these results be supported by RCT studies conducted in a variety of languages before definitive conclusions can be drawn on this issue.

3.2.6. Morphological and Syntactic Receptive Skills

In Law et al.’s [ 52 ] study, there are no indications of the effectiveness of interventions aimed at receptive syntax.

One of the studies already described [ 71 ] investigated the effectiveness of a training based on the “Competing Sources of Input” (CSI) hypothesis. It was also aimed at receptive grammar skills, in particular the understanding of questions with the present and past auxiliary (for a description of the study see the previous section). The results showed that understanding questions was better in children that were receiving a therapy based on the CSI hypothesis (with contrasts with respect to verb time) than in controls (where the stimuli are based on semantic contrasts). This hypothesis is closely linked to the acquisition of English grammar.

The information that is related to this area seems to be insufficient to draw any conclusions regarding the effectiveness of interventions aimed at improving receptive morphological and syntactic skills.

3.2.7. Narrative Skills

Law et al.’s [ 52 ] review did not investigate the effectiveness of interventions on narrative skills.

Only two RCT studies specifically evaluated the effectiveness of a narrative skill intervention. One was Maggiolo et al.’s study [ 79 ], which assessed the effectiveness of a program aimed at stimulating narrative skills based on the formal organization and content of the narrative. The experimental intervention consisted of three phases, i.e., interaction activities with the child, development of the experimental program, and interactive storytelling. The experimental program was structured into five mini-programs, i.e., temporal relationships, causal and purpose relationships, story presentation, storytelling, and storytelling structure. In the experimental group, significant differences were observed before and after the intervention in both the content and form of the story. In particular, the performance in causal and temporal relationships during the organization of the narrative content significantly improved, while no pre-post intervention differences were observed for the purpose relationships. No significant differences were observed within the control group. Note that the two groups were not directly compared.

Dawes et al.’ study [ 80 ] aimed to develop, test, and evaluate a small-group intervention targeting oral inferential comprehension within a book sharing context for 5- to 6-year-old children with DLD. Inferential and literal comprehension were both measured using a new methodology of assessment. Children were randomly allocated to one of two intervention groups: inferential comprehension group (ICI: N = 19) or phonological awareness control group (PA: N = 18). The mean comprehension scores prior to intervention were not significantly different for the two groups of children. When compared to the control PA group, the participants in the ICI group demonstrated a significant increase in the inferential comprehension scores from pre- to post-intervention, which was maintained over time. In addition, the ICI group scored significantly higher than the PA group for inferential comprehension on a post-intervention generalization measure. The results also demonstrated significant improvements at the individual level. No significant difference between the two groups for literal comprehension scores emerged at any assessment point.

The available information is still limited, but both studies examined reported clear increases in the comprehension of causal and temporal links after the intervention. Therefore, it appears to be important that these findings be confirmed and substantiated in further research.

3.2.8. Meta-Phonological Skills

The Law review [ 52 ] did not investigate the effectiveness of interventions on meta-phonological skills. Of the RCTs included, two specifically evaluated the effectiveness of different interventions on meta-phonological skills in children with DLD. A brief description of the individual studies follows.

The study conducted by Hesketh et al. [ 81 ] evaluated the effectiveness of specific training on meta-phonological skills through awareness tasks with phonemes and syllables. Specifically, the tasks first focused on syllables and rhymes, then on the recognition of the first and last phoneme of the word, and, finally, on the phonological manipulation of adding or deleting phonemes in the word. The children in the experimental phonological awareness (PA) intervention were compared to a control group that received a language stimulation (LS) program with activities of linguistic comprehension, knowledge of writing, verbalization of emotions, and development of vocabulary and semantics. No significant difference was found between the two groups for rhyming knowledge; on the contrary, a difference emerged regarding the ability to isolate, segment, and manipulate phonemes, as well as to add and suppress phonemes, in favor of the PA group. However, the results should be interpreted with caution, because of the large variability within the experimental group (e.g., for the two most advanced tasks, segmentation and addition/suppression, only a small minority of children showed improvements). Furthermore, only children with an adequate cognitive level showed that they benefited from the intervention: in fact, cognitively weaker children did not benefit, even after an intensive period of intervention.

Hund-Reid and Schneider’s study [ 82 ] evaluated the effectiveness of training on phonological awareness and grapheme-phoneme correspondence in preschool children. The intervention that was chosen for this study was the “Road to the Code”. This is a phonological awareness program for young children [ 96 ]. It is based on principles that include, in each session, explicit teaching of one or two types of phoneme manipulations (e.g., initial sound isolation and/or initial sound identification) and fusion and segmentation, as well as sound-symbol awareness activities (manipulation of phonemes with letters). The experimental group showed significantly greater improvement than the control group on the measures of phonemic fluency, phonemic segmentation, and non-word fluency. These gains were maintained, even one month after the intervention. Other aspects were also evaluated, such as the knowledge of writing and speed of reading letters (that were not among the outcomes defined in the research protocol carried out for this Consensus Conference) for which no significant differences were found.

Overall, there is still limited information on the effectiveness of interventions on meta-phonological skills in children with DLD; however, the results of the two reviewed studies indicate this as a potentially interesting area of intervention. Further work is warranted, possibly also examining languages other than English.

3.2.9. General Language Skills

Law’s review found only one study that aimed at training general linguistic skills with overall non-significant results.

Six of the RCTs included assessed the effectiveness of different interventions on language skills in general. A brief description of the individual studies follows.

In Roberts and Kaiser’ study [ 83 ], the effect of the “enhanced milieu teaching” (EMT) intervention that was carried out by parents (specifically trained by therapists and educators) on receptive and expressive language skills was evaluated. The intervention included four phases: setting the basics for communication; shaping and broadening communication; time delay strategies; and, finally, prompt strategies. The children in the treatment group showed higher gains in both expressive and receptive vocabulary than the control group. A comparison with the typically developing children showed that both groups with DLD continued to have significantly poorer language skills. However, when compared to the untreated children, the treated ones managed to grow at rates similar to those of children with typical development during the intervention. These results were considered to be preliminary, since the group size was not only small, but was a sub-sample of a larger study. Long-term outcome measures were also lacking.

Wake et al.’s studies [ 84 , 85 ] assessed whether an intervention on a population of four-year-old Australian children with language deficits could improve language outcomes and associated outcomes. Two-hundred four-year-old children already included in two previous studies (“Let’s Learn Language” and “Let’s Read” [ 97 , 98 ]) were selected (179 actually completed the study—91 in the experimental group and 88 in the control no-treatment condition). The children in the experimental group benefited from a home intervention aimed at promoting narrative skills, vocabulary, grammar, phonological awareness, and pre-reading skills, with a program that included 18 sessions distributed in three blocks of six weeks. This was characterized by: (a) a short review of the previous week; (b) activities introduced by the researcher directed towards the child; (c) activities for parents and children to be carried out together with the support of the researcher; and, (d) activities for home practice. The parents were then asked to talk to the child adopting specific language, to use a storybook, and to write down the activities in a diary. The control group, on the other hand, did not carry out any intervention.

At the five-year evaluation [ 84 ], significant improvement was found in the experimental group when compared to the control group for phonological awareness and graphemic recognition, but not for verbal production and understanding. A very positive perception by parents and a favorable cost–benefit ratio emerged. At a six-year assessment [ 85 ], improved language skills were found in both groups without significant inter-group differences. A significant improvement in phonological processing skills remained in the experimental group. In Wake et al.’s study [ 85 ], the authors reported that it was possible to implement relatively low-cost interventions with non-specialized personnel. However, no evidence emerged that this intervention actually improved outcome more than typical development. Finally, limitations also emerged: only a small number of families were at a disadvantaged socio-economic level and most of the language drops were mild. Therefore, it is not clear what results would have emerged in the case of children with a more compromised linguistic background.

The Wilcox et al.’s study [ 86 ] tested the efficacy of the Teaching Early Literacy and Language (TELL) curriculum [ 99 ] that was provided by preschool teachers. Ninety-one teachers were randomly assigned to TELL curriculum or Business-as-usual (BAU) contrast condition. Children with speech and language impairment in the experimental classes received, with the whole class, supportive and explicit teaching practices for oral language and early literacy skills. The authors did not find significant differences in the performance between TELL and BAU classes at a standardized assessment of receptive and expressive skills, phonological processing and awareness, and letter knowledge, but only in the curriculum-based measures (oral language and early literacy skills targeted in TELL program). They observed that BAU teachers also provided vocabulary and early literacy instruction and they concluded for the TELL efficacy for improving targeted oral language and early literacy skills.

Two studies [ 87 , 88 ] were derived from Loo et al.’s [ 79 ] review. The purpose of Gillam et al.’s study [ 87 ] was to determine whether a computer instructional program that was designed to improve auditory temporal processing skills (Fast ForWord-Language—FFW-L [ 100 ]) was more effective than other types of intervention for improving language and auditory processing in children with DLD. Two-hundred and sixteen children were randomly assigned to one of the four arms of intervention (see Appendix B for details). The children in all four arms made significant improvement in auditory processing, receptive and expressive language measures from pre- to post-testing, as well as in the follow-up three and six months later. Nevertheless, the children who received FFW-L did not do better than children in other interventions of equal intensity and primary outcome.

In Cohen et al.’s study [ 88 ], seventy-seven children with receptive-expressive specific language impairment were randomly assigned to Fast ForWord intervention [ 100 ], an alternative computer-based intervention or to a control group. After six weeks of computer games exposure at home, the improvements in expressive and receptive language performances of children in the experimental groups did not exceed those of the control groups.

Some of the studies on the effectiveness of different interventions on general language skills are now available, and they indicate this as a promising area of investigation. However, results appear to be mixed with several studies showing negative results.

4. Discussion

The main aim of the present review was to identify the most effective treatments to adopt for children with DLD. Because most of the studies aimed to verify the effectiveness of interventions for specific language skills, the analysis of the literature was organized according to the target language area. In this regard, the results of the new RCT studies that were identified after Law et al.’s [ 52 ] review partially confirm the indications already published and allow formulating some new hypotheses of effectiveness.

The interventions aimed at phonological, lexical, morphological and syntactic expressive skills are those most studied, presumably because these verbal components are easily identifiable and during treatment can be isolated from other aspects of language. The proposed activities are limited to these specific skills and the changes obtained can be verified through standardized scales of measurement. In addition, children with DLD who have a language profile that is limited to these skills are the most numerous. They are easily identified by non-specialist healthcare staff, school staff, and caregivers, and therefore represent a high percentage of access and demand for care by specialist services.

From the analysis of the studies included in this review, we can derive some general indications regarding the treatment of children with DLD. We have verified the effectiveness of intensive interventions based on the treatment with pairs of maximally contrasting phonemes, auditory discrimination, and phonological awareness. Regarding the phonological expressive component, a direct intervention, limited in time but intensive (i.e., three times a week), which includes auditory discrimination activities and it is based on contrast in traits, can bring significant improvements [ 63 , 64 ] that are maintained in the medium term.

Additionally, with regard to morpho-syntax, some of the strategies can be considered effective, such as recast or reformulation of the child’s production by the adult in conversation [ 70 ]. The same strategy is also proposed in tasks of storytelling that involve retelling stories [ 71 ]. Other valid strategies are that of cueing, i.e., providing suggestions to the child to try to stimulate the correct production [ 72 ], and that of the auditory prompt, i.e., explicit suggestions that are related to the grammatical rule [ 76 , 77 ]. Despite the effectiveness of these strategies, analysis of the studies has not allowed us to derive useful indications to define which of these could be the most appropriate in the area of treatment.

Furthermore, some of the studies have provided support for the effectiveness of treatments for meta-phonological and narrative skills [ 79 , 80 , 81 , 82 ]. The results appear to be related to the progress of research on the role of these skills in the language development of children with DLD.

New studies have also emerged that investigate interventions that aimed at developing general communicative language competence. These interventions are carried out at home, are mediated by parents, and are under the supervision of the clinician [ 83 , 84 , 85 ]. The interesting indication in these studies is that they are not only aimed at younger children with language delay, but that the indirect intervention also continues for children with DLD. These interventions are aimed at encouraging that more language skills be carried out in the contexts of the child’s life (at home and/or at school), so they can be considered to be more ecological, and the results could be easily generalized. The very recent study conducted by Wilcox (86) reported the efficacy of a teaching curriculum provided by teachers in improving oral language and early literacy skills of preschooler children with DLD. This study shows that even a systematized teaching program conducted in the school setting by teachers can promote a general improvement in communicative language abilities of preschool children with DLD, even if such an improvement is not evidenced at a standardized assessment of different areas of language. Still, one must add that results on general language skills are mixed, with several studies showing inconclusive findings (e.g., [ 87 , 88 ]). Thus, while the reviewed studies raise the interest in this type of intervention, the overall picture of findings is too scattered to be able to draw a firm conclusion on the effectiveness of intervention on general language skills.

Relatively little evidence of the strategies or techniques aimed at improving receptive language skills has been identified. One possible interpretation of the lack of results could be related to the complexity of interventions that aimed at verbal comprehension, i.e., it is difficult to isolate the individual receptive skills and free them from more general skills, such as pragmatic skills or semantics, creating activities that aimed at specific receptive skills in the rehabilitation setting. In addition, although children with a disorder that also affects language comprehension have a more severe clinical picture than children with only expressive DLD and a worse prognosis, they are numerically fewer. Therefore, it may be difficult to find enough children for an RCT study. Finally, the intervention may be less (or not) effective, also because, in some cases, weaknesses or deficits in comprehension (particularly lexical, morphosyntactic, and narrative) are associated with weak (even if within low-normal limits) cognitive skills.

Still, some recent studies have shown promising results. Regarding phonological skills, Roden et al. [ 68 ] demonstrated that the Auditory Stimulation Training with Musical material significantly improves working memory, phonemic discrimination, and speech perception in preschoolers with DPL. In evaluating the efficacy of an intervention targeting oral inferential comprehension within a book sharing context for preschoolers with DLD, Dawes et al.’ study [ 80 ] reported significant and sustained improvement in the group of children with DLD in inferential skills. Overall, the evidence on the effect of intervention on receptive phonological and on the comprehension of narratives skills is too limited to draw firm conclusions, but the quoted studies underscore the interest in further pursuing this area of research.

With respect to the other variables that are necessary to provide indications for treatment, such as the setting, frequency and duration of interventions, the way the results are evaluated, the age of the child, and the long-term effects of the intervention, the studies taken into consideration do not allow for us to draw firm conclusions. This is due to both the extreme heterogeneity across studies and the fact that these variables are often not described explicitly and exhaustively by the authors.

The risk of bias was high in some studies and unclear in most studies (particularly bias due to method of randomization, blinding of participants and personnel, and selectivity in the publication of results), thus decreasing the certainty in results. The fact that several studies examined relatively small samples of children is also of note; this may be problematic, particularly in the case in which a more comprehensive linguistic deficit is present (i.e., both expressive and receptive), which is typically associated with larger individual differences. These considerations indicate the importance and urgency that standards of RCT reporting will be improved in future research.

Regarding the transferability of the results to different languages as compared to the one in which the intervention is delivered, it appears that the specificity of the phonological repertoire and of the morphological and syntactical rules always deserves a reflection on the implicit differences between languages. Thus, for some language levels, it is possible to generalize the results, while, for others, it is necessary to modify the verbal stimuli of the treatment by adapting them to the linguistic context while maintaining techniques and strategies.

5. Implications for Clinical Practice and Research

The reviewed evidence highlights the importance of carrying out timely assessments of linguistic (and, in particular, phonological and morphosyntactic) skills in pre-school children, so as to provide, if necessary, targeted treatment before the start of primary school, while considering the importance of these skills for future school learning.

6. Limitations

There seems to be two general problems in analyzing the literature on interventions on DLD. On one hand, there are many more studies available for expressive than receptive disorder. We have noted above that this, in turn, may be due to the asymmetry in targeted children between these two types of disturbances. Yet, there is a risk here to consider that the interventions on expressive disorder are more effective, simply because more studies were carried out. Only further research on receptive disorder will allow for us to reach more definite conclusions on this point. On the other hand, most reviewed studies concern English and research on other languages is scattered. Accordingly, there is a need for systematic research in a wide range of other languages, particularly in the areas (such as morpho-syntax) where linguistic differences are more marked.

7. Conclusions

The present systematic review provides up-to-date information on the effectiveness of linguistic interventions for children with DLD. Evidence indicates that early intensive interventions in three to four-year old children are effective in the area of phonological expressive skills with acquisitions being maintained in the medium term. Some effectiveness of interventions over morpho-syntactic skills and, to some extent, on meta-phonological and narrative skills, was also detected. By contrast, there is fewer evidence that interventions on phonological receptive skills or receptive vocabulary are effective, but some recent studies raise the interest in further pursuing this area of research. A number of trainings aimed at general linguistic skills; the results are mixed, which makes it difficult to draw a definite conclusion, although there are indications that this may be a promising area of further investigation. Most research is carried out in English-speaking children, indicating the importance of studies in other languages.

Overall, information on linguistic interventions is quite different, depending on the linguistic skills investigated, indicating the need of further RCT studies in this area. Nevertheless, the currently available information indicates the importance of implementing timely assessments of linguistic skills and, whenever appropriate, targeted treatment.

Acknowledgments

The following scientific and professional societies also contributed to the Consensus Conference with funding and/or their referents, listed below, who discussed together the formulation of the clinical questions, the criteria for inclusion and exclusion of studies, and the findings emerged from the review of the literature: AIP (Associazione Italiana di Psicologia; Eng transl.: Italian Association of Psychology) represented by Pierluigi Zoccolotti and Olga Capirci; AIRIPA (Associazione Italiana per la Ricerca e l’Intervento nella Psicopatologia dell’Apprendimento; Italian Association for the Research and Intervention in the Psychopathology of Learning): Laura Bertolo and Barbara Carretti; AITNE (Associazione Italiana Terapisti della Neuro e Psicomotricità dell’Età Evolutiva; Italian Association of the Therapists of Neuro and Psychomotricity in Development): Maria Paola Colatei; ANUPI TNPEE (Associazione Nazionale Unitaria Terapisti della Neuro e Psicomotricità dell’Età Evolutiva; National Unitary Association of Therapists of Neuro and Psychomotricity in Development): Fiorenza Broggi; COSALING (COordinamento Società e Associazioni di LINGuistica; Coordination Societies and Association of Linguistics): Maria Elena Favilla and Maria Teresa Guasti; Istituto di Scienze e Tecnologie della Cognizione- Consiglio Nazionale delle Ricerche (Institute of Cognitive Sciences and Technologies, CNR, Italy): Maria Cristina Caselli and Pasquale Rinaldi; IRCCS E. Medea Associazione La Nostra Famiglia (Scientific Institute of Recovery and Care “IRCCS E. Medea”): Maria Luisa Lorusso and Andrea Marini; IRCCS Fondazione Stella Maris (Scientific Institute of Recovery and Care Stella Maris Foundation): Daniela Brizzolara and Anna Maria Chilosi; SIAF (Società Italiana di Audiologia e Foniatria; Italian Society of Audiology and Phoniatrics): Nicola Angelillo; SIFEL (Società Italiana di Foniatria e Logopedia; Italian Society of Phoniatrics and Speech Therapy): Antonio Schindler and Marina Tripodi; SIN (Società Italiana di Neonatologia; Italian Society of Neonatology): Nadia Battajon; SINPIA (Società Italiana Neuropsichiatria dell’Infanzia e dell’Adolescenza; Italian Society of Neuropsychiatry of Infancy and Adolescence): Roberta Penge and Cristiano Termine; SSLI (Società Scientifica Logopedisti Italiani; Scientific Society of Italian Speech Therapists): Luisa De Gasperi and Letizia Michelazzo. We are also grateful to the experts, who carried out the evaluation of the literature, the members of the Officina Napoli Cochrane who provided methodological support for literature evaluation, Federica Brancati, who has been in charge of the Organizational Secretary, and the members of the Jury who contributed to the discussion and writing of the Recommendations of the CC (information available at www.disturboprimariolinguaggio.it ; accessed on 14 February 2021).

Appendix A. Search Strategy for PubMed

Appendix B. Characteristics of the RCT Studies Included in the Systematic Review

Author Contributions

M.C.C., V.C., A.G.D.C., G.D.C., B.D.C., R.P., S.R., and P.Z. worked on the selection and analysis of studies and extrapolation of critical data; S.R. coordinated the research group; M.V.D.M. and S.V. provided methodological support; M.C.L., S.D., A.S., A.G.D.C., and T.R. promoted, followed and discussed all the phases of the Consensus Conference (CC) process on Primary Language Disorder and prepared the final documents for its accomplishment; S.R., A.G.D.C., and P.Z. prepared the first draft of the manuscript; all authors read, revised and approved the paper. All authors have read and agreed to the published version of the manuscript.

The present review was carried out as part of a Consensus Conference (CC), held in Italy, on the diagnosis and treatment of children with DLD (information on the CC is available at www.disturboprimariolinguaggio.it 30 December 2020). The Conference was organized and partially financed by CLASTA—Communication & Language Acquisition Studies in Typical & Atypical Populations, represented by Maria Chiara Levorato, Simonetta D’Amico and Alessandra Sansavini; and FLI—Federazione Logopedisti Italiani (Italian Federation of Speech Therapists), represented by Anna Giulia De Cagno and Tiziana Rossetto. FLI participated also with Manuela Pieretti.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Data availability statement, conflicts of interest.

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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REVIEW article

Behind the scenes of developmental language disorder: time to call neuropsychology back on stage.

• 1 National Research University Higher School of Economics, Moscow, Russia
• 2 Behavioural Science Institute, Radboud University Nijmegen, Nijmegen, Netherlands
• 3 Royal Dutch Kentalis, Sint-Michielsgestel, Netherlands

Although the Developmental Language Disorder (DLD), also known as Specific Language Impairment in children has been the focus of unceasing scientific attention for decades, the nature and mechanisms of this disorder remain unclear. Most importantly, we still cannot reliably identify children requiring urgent intervention among other ‘late talkers’ at an early age and understand the high prevalence of comorbidity with psychiatric phenomena such as Autism Spectrum Disorder. One of the main reasons for this is the traditional ‘diagnosis-by-exclusion,’ resulting in heterogeneity of the DLD population. This paper proposes an alternative approach to the diagnosis, treatment and research of DLD, claiming that it is these children’s multiple deficits in neuropsychological development, which impede the spontaneous acquisition of their first language. Specifically, this review of the state-of-the-art in DLD research demonstrates deep and systematic interconnections between the speech and other higher cognitive functions developing in early childhood, including perception, attention and executive functions. In the proposed framework, speech is, therefore, considered as one of neuropsychological abilities, and the delay in its development is explained by other neuropsychological deficits, resulting in highly individual clinical profiles. By considering DLD as a complex neuropsychological syndrome, whose successful treatment depends on a holistic approach to diagnosis and intervention, we may significantly increase the efficacy of speech therapy, and also better understand the flexibility of the developing brain, its compensatory mechanisms and hence the comorbidity of DLD with psychiatric symptoms. Implications for using this paradigm in future scientific research are discussed.

Introduction

Developmental Language Disorder (DLD) is characterized by the absence of speech in children despite their normal non-verbal IQ, no primary physical disabilities, neurological disorder or mental illness ( Leonard, 2008 ; Reilly et al., 2014 ; Bishop et al., 2016 , 2017 ). It is observed in approximately 5–10% of the population ( Tomblin et al., 1997a ; Law et al., 2000 ), and possibly because of the high proportion of children suffering from DLD, this disorder has long been the focus of attention in scientific research (e.g., Laurence and Karla, 1981 ; Rice and Wexler, 1996 ; Cleave and Rice, 1997 ; Leonard et al., 1997 ; Conti-Ramsden et al., 2001 ; Rice and Wexler, 2001 ; Rice et al., 2004 ; Marinis, 2011 ; Henry et al., 2012 ; Archibald et al., 2013 ; Kapalková, 2013 ; Vissers et al., 2015 ; Vissers and Koolen, 2016 ; Tomas et al., 2017 ) and clinical studies (e.g., Ukoumunne et al., 1999 ; Yoder and McDuffie, 2002 ; Law et al., 2003 ; Warren et al., 2007 ; Strong et al., 2011 ; Zeng et al., 2012 ; Smith-Lock et al., 2013 ; Law et al., 2017 ). It has been shown that DLD can be reliably diagnosed after the age of 4 years ( Whitehurst and Fischel, 1994 ; Paul, 1996 ; Rescorla et al., 2000 ) and that it can be roughly characterized as a lag of about 2 years in the development of language abilities ( Rice et al., 2006 ). A recent Delphi Consensus Study has additionally pointed out some specific indicators of atypical language development in 4–5-year-old children, including inconsistent or abnormal verbal interaction and at most three word utterances ( Bishop et al., 2016 , 2017 ). Importantly, children diagnosed with DLD as preschoolers later on often have difficulties in their social-emotional development ( St Clair et al., 2011 ; Vissers and Koolen, 2016 ; Forrest et al., 2018 ) and they also demonstrate lower levels of school performance. The latter, at least in part, can be attributed to the fact that a large proportion of children with DLD also develop dyslexia ( Bishop and Snowling, 2004 ; Rakhlin et al., 2013 ). It has further been shown that poor expressive abilities in early childhood are the best predictor of reading problems and dyslexia in school-aged children ( Lyytinen et al., 2015 ; also see discussion in Eklund et al., 2018 ). It appears, therefore, that these reciprocal connections between children’s limited expressive abilities during preschool years, and their reading difficulties and poorer academic performance at school, place children with DLD at a further disadvantage compared to their peers.

Over the past several decades both genetic ( Bishop et al., 1995 , 2006 ; Tomblin et al., 1997b ; Dale et al., 1998 ; Tomblin and Buckwalter, 1998 ; Bishop, 2009 ; Graham and Fisher, 2013 ; Rice, 2013 ) and environmental risk factors for DLD ( Tomblin et al., 1997b ; Bishop, 2009 ; Law et al., 2012 ) have been identified. Despite years of research, however, the underlying causes for this disorder are still not understood. The most serious problem in this respect poses the great amount of heterogeneity observed in this population, which suggests that DLD is probably not a single type of disorder, but an umbrella term for a variety of deficits in the domain of language acquisition ( Bishop, 1994 ; Conti-Ramsden and Botting, 1999 ). Purely linguistic accounts of DLD claim that this disorder is specific to language, suggesting that other neuropsychological processes remaining largely intact ( Rice and Wexler, 1996 ; van der Lely, 2005 ; Stavrakaki, 2006 ; Rothweiler et al., 2012 ). These linguistic approaches focus on establishing the various clinical markers of DLD in the language domain, which could be targeted during speech screening and intervention.

Alternative accounts of DLD have observed that children suffering from this impairment often have additional neuropsychological deficits accompanying their language problems. However, there has traditionally been a strong tendency to search for a single deficient neuropsychological mechanism underlying DLD, and thus the main body of research has compared children with and without DLD on the basis of either their working memory (WM) capacity ( Gathercole and Baddeley, 1990 ; Bishop et al., 1996 ; Archibald and Gathercole, 2006 ; Falcaro et al., 2008 ), or auditory perception abilities ( Tallal and Piercy, 1973 ; Bishop et al., 1999c ; Wright et al., 2000 ; Ziegler et al., 2005 ), or sustained attention ( Spaulding, 2008 ; Finneran et al., 2009 ; Ebert and Kohnert, 2011 ), etc. In contrast, it has recently been put forward that DLD is not only closely associated with neuropsychological deficits, but occurs when at least two cognitive processes are disrupted ( Bishop, 2006 ). This observation is in line with what has long been claimed by the proponents of neuropsychological approach to speech pathology going back to the 1930’s ( Vygotsky, 1934 ) and later expanded in the 1950–1960’s ( Luria, 1962 , 1966 ). Neuropsychology is concerned with the ‘behavioral expression of brain dysfunction’ ( Lezak et al., 2004 ) and it thus suggests deep interconnections between the various higher cognitive processes, including, for example, language and executive functions (EFs). Within this framework, the causes underlying the observed behavioral problems are thus thought to be rooted in multiple neurophysiological deficits.

In the speech and language domain, therefore, the ability to spontaneously acquire a language relies on the child’s neuropsychological skills, and thus the absence of speech needs to be considered as a symptom of their underdeveloped neuropsychological functions rather than an isolated deficit. This is supported by empirical evidence showing that language learning deficits are often observed across different clinical populations, including children with hearing loss ( Briscoe et al., 2001 ; Moeller et al., 2010 ), children with ADHD ( Geurts and Embrechts, 2008 ; Green et al., 2014 ), Autism Spectrum Disorder ( Koolen et al., 2012 ) as well as those with mental retardation ( Marrus and Hall, 2017 ) or cerebral palsy ( Hustad et al., 2014 ). In these groups of children the absence of speech is clearly secondary to another pathology, such as impaired auditory perception in children with hearing loss, or impaired executive control and social-emotional deficits in the ADHD and Autism Spectrum Disorder. What is not so clear is whether language deficits in DLD are also secondary – perhaps, not to a single primary disorder, but to a combination of underdeveloped higher cognitive functions. If this is the case, then our aim should be to identify at least some typical combinations of neuropsychological deficits in children with DLD and focus on the associations between these children’s neuropsychological profiles and their corresponding patterns of language-learning difficulties. Despite the existing empirical evidence, there is currently no reliable method correlating the child’s neuropsychological and language profiles. However, it seems that targeting those primary deficits in cognitive processes, which impede the child’s spontaneous acquisition of their first language, would be the first step toward increasing the efficacy of assessment and intervention. The following section of the paper, therefore, gives an up-to-date overview of what is known about cognitive performance by children with DLD.

Perception is a multi-dimensional ability, which can be explored not only across different domains (e.g., auditory, visual, tactile, etc.), but also in terms of the types of information processed within each domain. For the purposes of this paper, we will focus on the various aspects of auditory perception, since the perception in the visual domain appears to play a secondary role in the acquisition of oral speech ( Levina, 1951 ; Guenther and Hickok, 2015 ; Guenther, 2016 ). It has long been observed that at least some of the children with severe language-learning problems demonstrate deficits in their auditory perception skills. Specifically, back in early 1950’s it has been shown that despite these children’s normal hearing abilities, they fail to perceive linguistically meaningful contrasts ( Levina, 1951 ). In 1970’s, it has first been proposed that the perceptual deficit might be more generic in its nature and that children with DLD have difficulties in perceiving the various acoustic properties of non-verbal auditory signals as well ( Tallal and Piercy, 1973 ; Tallal et al., 1985 ). It has also been found that the skills in perceiving non-verbal auditory information can be trained, thus improving overall language abilities ( Tallal et al., 1996 ).

Both verbal and non-verbal auditory information can be described in relation to four physical parameters: duration, frequency, amplitude and phase. The first three are most relevant for studying atypical speech development because in the language domain they represent acoustic features used for discrimination of phonemes and words. Duration, or temporal processing, has been thoroughly examined in classical studies by Tallal and her colleagues, demonstrating that children with DLD have difficulties discriminating and reproducing tones of short duration, as well as determining the order of rapidly changing elements in a sequence ( Tallal and Piercy, 1973 ; Tallal, 1980 ; Tallal et al., 1985 ). A more recent study, reporting on a larger group consisting of 16 children with DLD, has expanded these observations by identifying two subgroups of children with DLD: those with poor and with normal temporal resolution abilities ( Ahmmed et al., 2006 ). In addition, the study discusses the interaction between these children’s temporal resolution and frequency perception abilities, suggesting a compensatory mechanism in children with DLD. Specifically, the subgroup of children who demonstrated poorer temporal resolution abilities showed greater frequency sensitivity.

The perception of frequency has also been studied independently of temporal effects, showing that children with DLD are less sensitive in perceiving voicing contrasts (e.g., /p/–/b/, as in a minimal pair p at – b at ) compared to their typically developing (TD) peers ( Ziegler et al., 2005 , 2011 ). These deficits in perceiving the elements of short duration and different frequency suggest that these children with DLD would have problems efficiently processing rapid speech, extracting phonological elements of short duration (e.g., grammatical morphemes in English, as in He run s ) and also in forming stable phonological representations of words, particularly if they form minimal pairs, as in s eal – z eal , or have similar sounding counterparts, as in agile – fr agile .

The research on the perception of amplitude focuses on the sensitivity to suprasegmental speech rhythm and stress patterns in children with DLD. Very few studies have explored these problems, and only in children’s perception of verbal signals. However, it has been shown that children with DLD tend to have decreased sensitivity to amplitude envelope rise time in interaction with both frequency ( Richards and Goswami, 2015 ) and duration ( Corriveau et al., 2007 ) of the signal. In the language domain, this means that children with DLD are likely to be less sensitive to lexical and phrasal stress and its violations.

However, the original hypothesis that speech deficits in children with DLD arise from their acoustic processing limitations has been challenged in more recent twin studies. Specifically, the authors have found no significant relationship between non-verbal and verbal auditory processing abilities in children with DLD ( Bishop et al., 1999a , b , c ; Bailey and Snowling, 2002 ). These findings have started a long-standing debate with some studies showing that at least some children with DLD have deficits in non-linguistic auditory perception and that their difficulties are not specific to language ( Wright et al., 1997 , 2000 ; Hill et al., 2005 ; Ziegler et al., 2005 , 2011 ; Corriveau et al., 2007 ; Vandewalle et al., 2012 ; Richards and Goswami, 2015 ). These disparate results are probably due to heterogeneity of the DLD population, particularly since the majority of these studies report on a small number of participants. It seems reasonable to assume that a proportion of children with DLD might have limitations in the auditory perception domain as the primary source of their language difficulties. It thus appears that neuropsychological assessment for children with DLD should include screening for possible deficits in auditory perception of both verbal and non-verbal signals.

Attention is essential for (language) learning because it serves as a ‘filtering system’ for the constant stream of input information, thus allowing to process only its relevant features. Attention is closely associated with both perception and EFs ( Johnston and Dark, 1986 ; Styles, 2006 ; also, see Figure 1 ). Specifically, like perception, attention can be an unconscious passive bottom-up process governed by our sensitivity to regularities in the input guided by our expectations and experience. For example, our attention is attracted by an unusual phenomenon, such as seeing a yellow leaf among green leaves on the pavement. This ability to unintentionally perceive regularities and deviations from patterns is closely associated with implicit learning (or so-called Statistical Learning) skills, which, due to their automatic nature, are believed to be one of the key leaning mechanisms across cognitive domains ( Turk-Browne et al., 2005 ; Endress and Mehler, 2009 ; Romberg and Saffran, 2010 ; Siegelman et al., 2016 ), functioning from infancy ( Saffran et al., 1996 ; Romberg and Saffran, 2010 ).

Figure 1. Neuropsychological perspective on cognitive and social functioning: the interplay between perception, attention and executive functions forms the foundation for language abilities. During social interaction, possible neuropsychological deficits are then behaviorally manifested through language as a proxy.

However, our attention can also be actively (i.e., consciously/intentionally) focused on some phenomenon, thus becoming a top–down process also known as attention control. For example, when we search for a birch tree leaf among other leaves on the pavement. Because attention control allows one to focus on a task and is involved in program selection, it is commonly listed among EFs ( McCabe et al., 2010 ; Najdowski et al., 2014 ; Drigas and Karyotaki, 2017 ; also, see a more detailed discussion of this topic in the following section of the paper).

Over the years, several models have been proposed to describe how these types of processes are carried out in the brain. One of the best empirically supported frameworks is the recent component theory of attention ( Mirsky et al., 1991 ; Gomes et al., 2000 ). This theory distinguishes several cortical sites orchestrating three types of behavior, associated with attention: alerting/arousal, orienting and executive attention ( Posner and Petersen, 1990 ; Mirsky et al., 1991 ; Petersen and Posner, 2012 ). Alerting refers to the general physiological readiness to perceive and process stimuli ( Gomes et al., 2000 ; Rueda et al., 2004 ; Petersen and Posner, 2012 ). It is associated with maintaining optimal vigilance and performance during tasks, and it is thus often recognized as an essential component of sustained attention ( Ballard, 1996 ; Styles, 2006 ; Finneran et al., 2009 ; Posner, 2012 ; Langner and Eickhoff, 2013 ; Fortenbaugh et al., 2017 ). Orienting involves shifting attention to endogenous or exogenous cues ( Corbetta and Shulman, 2002 ; Posner et al., 2006 ), which seems to be fundamental for unconscious/implicit learning mechanisms. Finally, executive attention refers to the ability to detect errors, resolve conflict among responses or inhibit responses ( Posner et al., 2006 ; Petersen and Posner, 2012 ). This form of behavior, which has been originally proposed as part of the attention network ( Posner and Petersen, 1990 ), is also often considered among EFs (see more on this in the discussion of inhibitory control in the “Executive Functions” section). In addition, in many models of attention executive attention is associated with selective attention, i.e., the ability to detect only relevant stimuli in the stream of input ( Moray, 1959 ; Johnston and Dark, 1986 ; Wood and Cowan, 1995 ; Stevens et al., 2006 ; Shinn-Cunningham, 2008 ; Getzmann and Näätänen, 2015 ).

Several studies have attempted to explore these problems from developmental perspective ( Ruff and Rothbart, 1996 ; Gomes et al., 2000 ; Rueda et al., 2004 ; Thillay et al., 2015 ; Suades-González et al., 2017 ), confirming that some types of attention like arousal and orienting systems form in early infancy, and finding mixed results for selective and sustained attention abilities ( Gale and Lynn, 1972 ; Swanson, 1983 ; Gomes et al., 2000 ). The former is probably indicative of the innateness of some automatic/unconscious (bottom–up) attention mechanisms, such as those involved in Statistical Learning. More cognitively demanding and complex types of behavior, including controlled selective and sustained attention or divided attention, which requires simultaneous processing of multiple streams of information ( Sohlberg and Mateer, 1989 ), are likely to develop gradually, with the maturation of the respective cortical areas. However, more research is needed to determine the developmental trajectories for these types of attention across auditory, visual and other domains (see for review: Gomes et al., 2000 ).

Regarding children with DLD, it seems that at least some of the bottom–up processes are impaired in this population. Specifically, the unconscious attention mechanisms associated with implicit learning tend to be weaker in these children compared to their TD peers (see for reviews: Lammertink et al., 2017 ; Zwart et al., 2017 , 2018 ). However, it is not clear whether these poorer abilities in perceiving statistical regularities are domain-specific (e.g., visual vs. auditory statistical learning skills); and also if there is a difference between verbal and non-verbal types of tasks within the auditory domain. More cognitively demanding types of attention have been studied less systematically in children with DLD, often producing disparate results. Recent meta-analysis on sustained attention abilities in the DLD population, however, supports the idea that these children tend to have deficits in sustained attention across auditory and visual modalities, and that larger effect sizes can be found for auditory (both verbal and non-verbal) stimuli ( Ebert and Kohnert, 2011 ). Similarly, in the selective attention domain, children with DLD demonstrate poorer performance during verbal and non-verbal auditory tasks ( Stevens et al., 2006 ), but not in visual tasks ( Spaulding, 2008 ).

To summarize, attention deficits could be one of the underlying causes of DLD. However, it is not yet clear how the various types of attention interact to meet cognitive demands for these children. For example, it is not known whether children with DLD develop some compensatory executive attention mechanisms if they have deficits in their implicit/unconscious attention abilities. In addition, more evidence is needed to understand the extent of their attention deficits across modalities, and also during more cognitively demanding types of tasks, involving alternating and divided attention.

Executive Functions

Executive functioning can be defined as a top–down control of cognitive processes for goal achievement. Executive control is necessary in the regulation of more automatic processes (thoughts, behavior, emotion) in the service of a goal or to adjust to changing circumstances ( Miyake et al., 2000 ). EFs are especially important in situations in which relying upon automatisms or impulses is unwise or even impossible, as in non-routine situations ( Diamond, 2013 ). There is controversy over the specific components of executive functioning and the way they relate to each other ( Barkley, 1997 , 2012 ; Miyake et al., 2000 ). A well-established conceptualization of EFs is Miyake’s model, which considers it as a unitary construct with three separable major components: inhibition of prepotent responses (inhibiting), shifting between tasks or mental sets (shifting) and information updating and monitoring of WM representations (updating).

Inhibition refers to the ability to deliberately suppress dominant or automatic responses and to resist interference of distractors ( Friedman and Miyake, 2004 ). Inhibitory control is important for learning as it helps maintain sustained and focused attention necessary for the acquisition of new skills and knowledge (see also section “Attention”). Inhibiting is also important for social functioning; for example, resisting impulses and temptations (e.g., waiting for your turn) is essential for establishing and maintaining social relationships ( Tangney et al., 2004 ).

Shifting between tasks or mental sets involves the disengagement of a task set and the active engagement of a new task set. Shifting also involves the ability to switch between operations or mental sets. The ability to shift is strongly related to cognitive flexibility. It is essential to social-emotional functioning as people bring their own goals, impulses, desires, and emotions into social situations, which makes every social situation unique and often very complicated and unpredictable ( Parsons and Mitchell, 2002 ).

Updating is the ability to actively manipulate the contents of WM and to monitor the incoming information with the aim of keeping track of which information is relevant and update items in WM with new, more relevant information. In conceptualizing WM, Baddeley’s multicomponent WM model is widely used. It comprises three subsystems governed by the central executive: the phonological loop, the visuo-spatial sketchpad, and the episodic buffer ( Baddeley, 2000 ). The former two are ‘slave’-systems responsible for temporary storage of verbal and visuo-spatial information. The episodic buffer is proposed to integrate representations from WM, long-term memory and language processing systems. WM is essential for learning and also for social functioning because it subserves temporal processing of social information during interactions, keeping social goals actively in mind, retrieving social information from long-term memory, and selecting an appropriate social response (see also Vissers and Hermans, 2018 ).

Executive functions have been discussed extensively in investigating DLD over the past decade. Deficits or delays in the development of EFs in children with DLD have been reported for many components of the executive system, but in some more than in others (e.g., Hill, 2004 ; Bishop and Norbury, 2005 ; Castellanos et al., 2006 ). Specifically, studies on inhibitory control processes have reported consistent results, showing that children with DLD tend to be impaired in their inhibiting abilities (e.g., Bishop and Norbury, 2005 ; Marton et al., 2007 ; Pauls and Archibald, 2016 ). In particular, children with DLD are more susceptible to distraction ( Lum and Bavin, 2007 ). Interestingly, this impaired performance of children with DLD has been described for the auditory distraction task regardless of whether a distractor was related or unrelated to the target stimuli. This suggests that children with DLD might have a generic distractor processing problem ( Victorino and Schwartz, 2015 ).

With respect to cognitive flexibility, findings are mixed. Thus, some studies on shifting have not found any deficits in children with DLD (e.g., Kiernan et al., 1997 ; Im-Bolter et al., 2006 ), others have brought to light attentional shifting problems in addition to decreased cognitive flexibility ( Marton, 2008 ). Overall, there is no consistent evidence indicating an impairment in shifting ability of the DLD population (e.g., Kapa and Plante, 2015 ), and more research is needed to explore the developmental trajectory of this ability and its role in language learning in both TD and DLD populations.

Studies on updating and WM in children with DLD have found limitations on both phonological and non-verbal WM tasks ( Marton and Schwartz, 2003 ; and see also Archibald and Gathercole, 2006 ; Bishop, 2006 ; Montgomery et al., 2010 ; Duinmeijer et al., 2012 ). In line with these findings, Im-Bolter et al. (2006) have found impairments in children with DLD in updating the general WM content. In contrast, some studies examining non-verbal updating ability in DLD show conflicting results. Specifically, several studies report similar visuo-spatial updating performance of children with DLD and their TD peers (e.g., Lum et al., 2012 ). However, recent meta-analysis on visuo-spatial WM in DLD ( Vugs et al., 2013 ) suggests that for this population WM deficits indeed extend to the non-verbal domain. Similarly, Henry et al. (2012) have observed differences in verbal and non-verbal updating ability between DLD and TD children after controlling for their non-verbal IQ, and also differences in non-verbal updating ability after controlling for their verbal IQ. This suggests that poorer non-verbal updating performance reflects a domain general updating deficit.

Importantly, it is often challenging to explore EFs in children under 4 years due to cognitive demands of the tasks. Thus, a lot of research in this area has so far focused on school-aged children, and more evidence is required to understand the role of EFs on language acquisition in children. Recent studies suggest that similarly to school-aged children with DLD, preschoolers with DLD tend to show difficulties in WM, inhibition and shifting, as revealed by both performance-based measures and behavioral ratings ( Vissers et al., 2015 ). However, it is not yet clear to what extent this relationship between the developing EFs and language abilities in children is reciprocal, and to what extent it might be causal. It thus seems particularly important to explore the interconnections between children’s individual language profiles and EFs during typical development and as a part of the diagnosis for DLD.

This paper aimed at bringing together findings from different areas of neuropsychological research, exploring DLD and its underlying causes. It focused on how the various higher cognitive processes, including perception, attention, inhibition control, mental flexibility and WM may affect the spontaneous emergence of speech. Despite the observed inconsistencies across individual studies, overall there seems to be a strong association between DLD and the deficits in higher cognitive processes essential for normal language acquisition and functioning. In particular, there appears to be a continuous interplay between perception, attention, EF and language across childhood (see Figure 1 for a schematic representation of this neuropsychological perspective on cognitive and social functioning). This cognitive interplay might underlie problems in communication and social-emotional functioning observed in many children with DLD. From here, we propose that DLD needs to be treated as a complex neuropsychological syndrome during diagnosis and therapy. Specifically, it appears that diagnosis will benefit from screening for possible neuropsychological deficits underlying DLD, and that targeting these impaired neuropsychological abilities is likely to complement and enhance speech intervention.

Several important theoretical questions remain open, however. First, it is not clear if there is a straightforward correlation and interaction between different modalities (e.g., auditory vs. visual attention deficits). Second, it is not known whether for auditory information these deficits are specific to linguistic input or are more generic in their nature (e.g., verbal vs. non-verbal WM limitations). Third, little is known about how the various types of neuropsychological deficits interact, and how this may reflect on the child’s individual linguistic profile. Also, more information is needed to better understand the developmental trajectories of the higher cognitive processes observed in TD children. Thus, for example, it is not known whether a child with normal language skills may have deficits in one or more higher cognitive abilities; and if so, what compensatory mechanisms they develop to prevent these neuropsychological impairments from leading to DLD. Finally, it seems essential to resolve the problem of heterogeneity in the language abilities observed in the DLD population, which is likely to be due to variability in types of neuropsychological deficits impeding the spontaneous emergence of speech in these children (see Box 1 ). Thus, matching their individual language and neuropsychological profiles during diagnosis would allow selecting a more effective intervention program that would target specific deficient cognitive ability [e.g., WM training, attentional training, perceptual (rhythmic/music) intervention etc.]. This would also be highly beneficial for research purposes – for identification of subgroups of children with DLD and accounting for their performance during cognitive and language tasks.

Clinical recommendations ‘To come to tailored assessment and treatment we recommend to assess every individual child with DLD neuropsychologically in addition to their language profiling. Next to linguistic abilities, one should at least zoom in on perception, attention and executive functioning (working memory, inhibition and flexibility). These cognitive domains should be brought to light both behaviourally (e.g., with behavioral rating scales) and cognitively (e.g., with cognitive tests). This is essential not only for clinical purposes (to come to tailored assessment and treatment), but also for experimental studies, when children with DLD are currently treated as a single population while they are likely to represent several clinical groups.’

Author Contributions

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

This work was supported by Royal Dutch Kentalis and the Russian Foundation for Basic Research (Grant 18-312-00188).

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors are very grateful to their wonderful philosopher and artist Jet Isarin for her beautiful visualization of the neuropsychological model of developmental language disorder.

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Keywords : Developmental Language Disorder, Specific Language Impairment, neuropsychology, executive functions, attention, perception

Citation: Tomas E and Vissers C (2019) Behind the Scenes of Developmental Language Disorder: Time to Call Neuropsychology Back on Stage. Front. Hum. Neurosci. 12:517. doi: 10.3389/fnhum.2018.00517

Received: 11 July 2018; Accepted: 07 December 2018; Published: 09 January 2019.

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Copyright © 2019 Tomas and Vissers. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Ekaterina Tomas, [email protected] Constance Vissers, [email protected]

† These authors have contributed equally to this work and share first-authorship

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• Study protocol
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• Published: 21 May 2018

Understanding developmental language disorder - the Helsinki longitudinal SLI study (HelSLI): a study protocol

• Marja Laasonen   ORCID: orcid.org/0000-0002-4628-4251 1 , 2 , 3 ,
• Sini Smolander 1 , 4 ,
• Pekka Lahti-Nuuttila 1 , 2 ,
• Miika Leminen 1 , 8 ,
• Hanna-Reetta Lajunen 10 ,
• Kati Heinonen 2 ,
• Anu-Katriina Pesonen 2 ,
• Todd M. Bailey 5 ,
• Emmanuel M. Pothos 6 ,
• Teija Kujala 8 ,
• Paavo H. T. Leppänen 11 ,
• Christopher W. Bartlett 12 ,
• Ahmed Geneid 1 ,
• Leena Lauronen 9 ,
• Elisabet Service 7 ,
• Sari Kunnari 4 &
• Eva Arkkila 1  

BMC Psychology volume  6 , Article number:  24 ( 2018 ) Cite this article

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Developmental language disorder (DLD, also called specific language impairment, SLI) is a common developmental disorder comprising the largest disability group in pre-school-aged children. Approximately 7% of the population is expected to have developmental language difficulties. However, the specific etiological factors leading to DLD are not yet known and even the typical linguistic features appear to vary by language. We present here a project that investigates DLD at multiple levels of analysis and aims to make the reliable prediction and early identification of the difficulties possible. Following the multiple deficit model of developmental disorders, we investigate the DLD phenomenon at the etiological, neural, cognitive, behavioral, and psychosocial levels, in a longitudinal study of preschool children.

In January 2013, we launched the Helsinki Longitudinal SLI study (HelSLI) at the Helsinki University Hospital ( http://tiny.cc/HelSLI ). We will study 227 children aged 3–6 years with suspected DLD and their 160 typically developing peers. Five subprojects will determine how the child’s psychological characteristics and environment correlate with DLD and how the child’s well-being relates to DLD, the characteristics of DLD in monolingual versus bilingual children, nonlinguistic cognitive correlates of DLD, electrophysiological underpinnings of DLD, and the role of genetic risk factors. Methods include saliva samples, EEG, computerized cognitive tasks, neuropsychological and speech and language assessments, video-observations, and questionnaires.

The project aims to increase our understanding of the multiple interactive risk and protective factors that affect the developing heterogeneous cognitive and behavioral profile of DLD, including factors affecting literacy development. This accumulated knowledge will form a heuristic basis for the development of new interventions targeting linguistic and non-linguistic aspects of DLD.

Background to the study

Language does not always develop as expected, which can have devastating effects on both individual and societal levels. Developmental language disorder (DLD, previously called specific language impairment, SLI) is a common developmental disorder comprising the largest disability group in pre-school-aged children. Approximately 7% of the population is expected to have DLD [ 1 ]. Somewhat surprisingly, DLD has received relatively little research interest compared to less prevalent disorders, such as autism spectrum disorders (ASD) and attention deficit/hyperactivity disorder (ADHD) [ 2 ]. Although DLD is diagnosed most often in childhood, the associated difficulties are not restricted to this developmental period. Rather, DLD also often leads to dyslexia [ 3 ] and it may continue to restrict the person’s social, academic, and occupational activities even beyond adolescence and into adulthood. For example, a recent study of adolescents in reform school found that poorer verbal skills were associated with elevated levels of later criminal behavior [ 4 ]. Further, the previous work of our research group has shown that 26% of adults with a childhood diagnosis of DLD in Finland are pensioned off and 19% live with their parents [ 5 ]. This truly highlights the long-term risk for social marginalization associated with DLD.

To cope with this risk caused by a developmental challenge, it is vital to understand better the interactions between harmful and protective factors that affect the developmental manifestation of DLD. However, at the moment, the specific etiological factors leading to DLD are not known. In many cases, developmental language difficulties are suggested to be caused by genetic factors [ 6 ]. At the neural level, perisylvian brain areas contributing to language processing are often affected [ 7 ]. However, the exact mechanisms that lead the neural abnormalities to cause DLD are not known. Presently, we do not even fully understand the range of cognitive or behavioral difficulties associated with DLD. For example, the cognitive difficulties have been suggested to span nonverbal as well as verbal domains, and the linguistic markers of DLD appear to vary from one language to another [ 8 ].

The genetic and neurobiological studies cited above suggest that DLD has a biological basis. However, language learning can be modulated also by, for example, reduced exposure to the language used in school and society. Of the population in Finland, 6.4% had a language other than Finnish, Swedish or Sami as their first language at the end of year 2016, and this percentage is rapidly growing [ 9 ]. Many of these are immigrants or people with immigrant background. Based on Finnish official statistics [ 10 ], one of the most significant predictors of successful employment for immigrants is an education acquired in Finland. Especially for bilingual children of immigrant families, language skills are the best predictors of successful educational attainment [ 11 ]. Naturally, also some of the bilingual children are expected to suffer from DLD. However, bilingual environment itself is not considered to be a risk factor for language impairment [ 12 ], and, thus, language impairment should be equally prevalent in monolinguals and bilinguals [ 13 ]. In contrast to this suggestion, of the children seen for the first time at the Audiophoniatric Ward for Children, Department of Phoniatrics, in the Helsinki University Hospital, a disproportionate 30–40% are multilingual. Although part of this amount may reflect a referral bias and challenges in diagnostics, it is also compatible with the possibility that the risk of language impairment, or especially severe language impairment [ 14 ], is elevated in bilingual and multilingual children compared to monolingual children. In annual follow-ups, the diagnoses of these bilingual children seldom change. This suggests that DLD does, indeed, explain their difficulties. This marked over-representation of bilinguals with suspected DLD warrants investigation of the underlying phenomena.

Summary of the existing literature

Psychosocial factors in dld.

The child’s proximal environment, e.g. parent-child interaction patterns, and his or her individual traits and characteristics may affect both language development and response to intervention. For example, the quality of mother-child interaction moderates the effects of a biological disadvantage on later cognitive functioning [cf. studies on low birth weight, 15 , 16 ]. In terms of temperamental traits, children with language difficulties have been shown to be less persistent in their temperament compared to typically developing (TD) peers [ 17 ]. However, to our knowledge there is no previous research on the effects of parent-child interaction specifically focusing on language development in DLD, nor has temperament been thoroughly assessed in a longitudinal setting.

Developmental language difficulties themselves may have a negative impact on the child’s self-esteem and well-being. Rescorla et al. [ 18 ] have shown that language delay is associated with social withdrawal already in toddlers, as assessed with the Child Behavior Checklist (CBCL) [ 19 ]. St Clair et al. [ 20 ] followed 7–16-year-old children with DLD with the Strengths and Difficulties Questionnaire (SDQ) [ 21 ] and found that during this time-period, social problems increased and emotional problems persisted into adolescence. In relation to the social problems, the previous work of our research team has shown that adults with a childhood history of DLD perceive many dimensions (usual activities, mental functioning, and speech) of their health-related quality of life (HRQoL) [ 22 ] to be poorer than that of the controls. This parallels with the fact that DLD adults of the study lived with their parents or were pensioned more often than the adult Finnish population on average [ 23 ]. We are not aware of any previous DLD research that has focused both on the etiological (e.g., temperament) and outcome psychological and psychosocial factors (e.g., well-being of the child). Recognizing these risk and protective factors and their consequences, both in the environment and within the child, would permit prevention.

Bilingualism and DLD

Differentiating DLD from TD in bilinguals is a challenging task for health care professionals. Lack of knowledge, normative data, and tools may often lead to over- or underdiagnosing. There are various suggestions for how DLD and bilingualism combine. Monolingual DLD and bilingual TD have been proposed to resemble each other in some ways, for example in terms of morphological forms used [ 24 ]. Bilingual DLD children have also been suggested to be affected by a double deficit [discussed in, 25 ], since they could suffer from both restricted cognitive (due to DLD) and restricted environmental (due to bilingualism) resources [see also, 26 ]. On the other hand, another recent suggestion is that although bilingual DLD children may suffer from restricted cognitive resources (similarly to monolinguals with DLD), the demands of their environment result in a “bilingual advantage” in, for example, executive functions [ 27 ]. Especially in the case of sequential bilingualism (L2 learning), it is suggested that various child-internal (e.g., first language, L1 typology, and child’s age) and child-external (e.g., amount of language exposure) factors play an important role in performance and development [ 27 ]. Despite of critically lacking information, such a large scale longitudinal study on 3–6-year-old bilingual DLD children has not been conducted.

Cognitive factors and DLD

Although DLD by definition means compromised skills in the language domain (domain-specific impairment), there is accumulating evidence that the difficulties of those with DLD may not actually be restricted to language, there instead being a domain-general impairment. In fact, nonlinguistic basic cognitive capacities are also likely to be involved, and some of these characteristics may well be shared across different languages. If this is so, new assessment and intervention possibilities could present themselves. Recent findings of domain-general capacities that might affect language development have been reported on different levels. At the etiological level, genetic factors behind DLD appear to affect not only language but also nonverbal ability [ 28 ]. At the cognitive level, there are several suggestions for nonlinguistic difficulties, for example, impaired general processing speed and short-term memory (STM) or working memory [ 29 , 30 ]. One other recent hypothesis at the cognitive level, as put forward by Ullman [ 31 ], Nicolson and Fawcett [ 32 ], suggests that DLD could result from a generalized difficulty in acquisition of automatic skills, including procedural learning. Procedural learning is typically implicit and refers to learning of habits, skills, and procedures [ 33 ] as opposed to knowledge that can be explicitly articulated. Procedural learning mechanisms might be linked to language development in complex ways. For example, both procedural learning and language development would be compromised if their underlying cognitive core capacities are impaired. Also, they could form a cluster of functions linked to one another in a correlative or causative way. Unraveling these relations would have far-reaching consequences for how specific we perceive various developmental and learning impairments to be and how those with difficulties should be supported.

Initial diagnoses of DLD are often complemented with findings of impairments related to literacy when the child reaches school age. In fact, reading disability or dyslexia is so common among individuals with DLD that it has been suggested to be another symptom of the same syndrome. However, there is controversy as to the extent and nature of overlap between DLD and dyslexia [ 34 ]. We have shown recently that difficulties of written language in adults (i.e., dyslexia) correlate with modality-general impairments in processing speed [ 35 ] and STM [ 36 ], and argued that these may both relate to underlying difficulties in the processing of information that requires attentional control of temporal binding. Another recent project led by Prof. Laasonen ( https://www.helsinki.fi/en/researchgroups/project-dyadd ) showed that adults with developmental dyslexia also have difficulties in nonverbal procedural learning [ 37 ]. Importantly, poor performance in these affected nonlinguistic areas of cognition was shown to be related to poor linguistic skills. As developmental dyslexia could be one of the possible developmental end-results of childhood DLD, it is vital to expand this research to DLD children, in order to validate the findings of the older age-groups in young children [ 38 ].

Electrophysiology in DLD

Continuous electroencephalogram (EEG) recording has been a routine procedure in DLD diagnostics. One of the reasons is the necessity to exclude serious conditions, such as the Landau-Kleffner syndrome [ 39 ]. Otherwise, the rationale for clinical EEG in DLD diagnostics remains unresolved. Some studies have found elevated amounts of epileptiform activity in EEG of children with DLD [ 40 , 41 , 42 , 43 , 44 ]. Other researchers have suggested that especially those with syntactic-phonological or syntactic-lexical difficulties would have abnormalities in continuous EEG recording [ 45 , 46 ]. To our knowledge, there is only one longitudinal study on clinical EEG in DLD [ 47 ]. It failed to find significant associations between original epileptiform EEG and later language development in a very small group of children. Thus, it remains unclear, whether children with DLD, in general, have abnormal EEG findings or whether the abnormalities are confined to a specific subgroup or if EEG has predictive value on DLD in a longitudinal setting. Finally, the mediating role of comorbid conditions has not been resolved. For example, developmental coordination disorder [ 48 ] and ADHD [ 49 ] have been associated with EEG abnormalities.

Genes and DLD

Developmental language difficulties are in many cases affected by genetic factors. Half of the children with DLD have relatives with language difficulties and the concordance rate for monozygotic twins is higher than that for dizygotic twins [ 50 ]. At least three different genetic loci (DLD1 at 16q, DLD2 at 19q, and DLD3 at 13q21) and two genes that are expressed in the brain (CMIP and ATP2C2 in chromosome 16) have been suggested to contribute to DLD [ 6 ]. The exact role of these genes is not known but, in their review, Li and Bartlett [ 6 ] suggest that they could contribute to phonological STM. Also other DLD candidate genes (e.g., CNTNAP2 and BDNF), have been suggested to contribute to STM as well as to difficulties in verbal comprehension and expression. Importantly, all four replicated genes involved in DLD aetiology, ATP2C2, BDNF, CMIP, and CNTNAP2, have common genetic variants that occur in persons of European ancestry. These genes have not been assessed in the Finnish population, which has some minor genetic differences from the rest of Europe due to the relatively small number of founding members of the Finnish population that migrated to present day Finland 4000 years ago. Further, more detailed information about different risk alleles’ contribution to specific cognitive and linguistic factors has not been conducted in a longitudinal setup, especially involving bilingual children.

We present here an ongoing project, the Helsinki Longitudinal SLI study (HelSLI, http://tiny.cc/helsli ) that investigates DLD in preschool children at the etiological, neural, cognitive, behavioral, and psychosocial levels of analysis with an aim to answer the many open questions and to increase our understanding of the multiple interactive risk and protective factors that affect the developing heterogeneous cognitive and behavioral profile of DLD. HelSLI study consists of five subprojects.

HelSLI-psychosocial

HelSLI-psychosocial investigates how the child’s psychological characteristics (i.e., temperament) and proximal environment (i.e., parent-child interaction) influence DLD and response to rehabilitation in a longitudinal setting. HelSLI-psychosocial investigates also how DLD relates to the psychosocial characteristics and well-being of the children. We hypothesize that both child temperament and parent-child interaction include risk and protective factors for language development, and that DLD itself is a risk factor for the long-term well-being of a child.

HelSLI-bilingual

The bilingual children of the current study are early sequential bilinguals who acquire Finnish as their second language not from the birth but early on in kindergarten. We use a two-way design (TD/DLD x mono/bilingual, that is, MonoTD, BiTD, MonoDLD, and BiDLD), longitudinal approach as well as consider age and exposure effects and their interaction. Thus, we are able to answer many of the open questions [ 25 ]. We hypothesize, based on the literature [ 12 , 26 , 51 ] and our preliminary data, that children with bilingual background will have poorer language performance compared to monolinguals when using tests developed for monolinguals but fewer comorbid characteristics. Possible bilingual advantage might be seen in compensating the hypothesized double deficit of restricted environmental resources and restricted cognitive resources. This advantage might prevent bilingual DLD children from falling behind their TD bilingual peers and could be observed in various cognitively demanding tasks included in the clinical neuropsychological battery and HelSLI-cognitive, and also in different linguistic areas at later stages of the longitudinal setting. We also hypothesize, since DLD and TD can resemble each other in bilingual setting, that it would be more appropriate to compare BiDLD children to BiTD children and not to MonoTD children when assessing developmental language disorder.

HelSLI-cognitive

In HelSLI-cognitive, we aim to test nonlinguistic factors that could potentially be used in prediction, diagnosis, and intervention of DLD across languages, in this case, auditory and visual STM and artificial grammar learning (AGL) [ 52 ]. We hypothesize, based on our own previous research and recent literature cited above, that DLD children will have more difficulties than TD children in the nonverbal tasks of STM and AGL across modalities, when required to maintain, chunk, manipulate, and learn patterns. In addition, we can explore whether any impairment in AGL can be identified to specific types of information, for example, high frequency bigrams vs. whole exemplars vs. long range associations.

To our knowledge, there is scarcely previous neurophysiological or functional imaging research on bilingual children with DLD [see, however 53 ]. Also, in case of studies on monolingual DLD children, most of the research has been conducted either with newborns, school-aged children, or adolescents whereas there is less research on preschool-aged children. The HelSLI-EEG sub-project thus focuses on identifying neurophysiological markers of DLD in monolingual and bilingual children with EEG and offers data on DLD children in the age range of 3–6 years – a time during which language skills develop rapidly but on which there is scarcely brain research. Both continuous clinical EEG and ERP recordings are being used. First, we aim to study, whether epileptiform activity is related to a specific cognitive impairment profile within DLD spectrum. Secondly, by ERP assessments, we aim to elucidate the cognitive dysfunctions in DLD at the levels of basic auditory processing, phonological processing, and STM as well as morphological processing. ERP assessments that are this wide-ranging have never been done in DLD research before. We preliminarily hypothesize that epileptiform abnormalities in clinical EEG are related to the severity of DLD in both mono and bilingual children. Based on previous literature on ERP indices in DLD, we expect to find attenuated MMN responses for tone frequency changes as well as consonant contrasts in syllable stimuli [ 54 ]. Importantly, we will be able to anchor these findings to other simultaneously measured linguistic and non-linguistic ERP contrasts, as well as to the detailed cognitive and linguistic behavioral profiles of individual children with DLD. In the framework of procedural learning impairment hypothesis, we expect to find indices that reflect neural dynamics of the acquisition of phoneme and morpheme sequences to be impaired in DLD.

HelSLI-genetic

HelSLI-genetic investigates the role of four known genetic risk factors (ATP2C2, BDNF, CMIP, and CNTNAP2) in DLD in the Finnish monolingual and bilingual populations. Should these genes be associated with DLD or related cognitive functions and neurophysiology in Finnish DLD cases, this will be the first such demonstration in this population, and these markers will be assessed for utility in predicting intervention outcomes. Also, these markers are of potential use as covariates for the analysis in the other subprojects, since the genetic markers may demarcate some error variance if multiple different DLD etiologies are, in fact, present. We hypothesize that language ability and more specifically STM (here also nonverbal) will be related to the genetic background in our sample.

Methods and design

Design and setting.

HelSLI study is realized at the Audiophoniatric Ward for Children, Department of Phoniatrics, Helsinki University Hospital. Healthcare professionals on the department work in multidisciplinary teams focused on the assessment and diagnosis of the children with DLD or suspected DLD. These include medical doctors specializing in phoniatrics, speech and language pathologists, neuropsychologists, occupational therapists, special education teachers, and nurses. Most of the DLD sample data was gathered alongside normal clinical work. For the HelSLI study participants, we formulated standardized clinical EEG, neuropsychological (Additional file  1 : Appendix 1) and speech and language assessment protocols (Additional file  2 : Appendix 2) that were applied for each incoming and eligible first-time child at the Audiophoniatric Ward for children, Department of Phoniatrics, Helsinki University Hospital, during years 2013–2015.

Data collection begun in January 2013. The total number of 3-to-6-year-old children with suspected DLD who entered the HelSLI study was 246 (three entry years, 2013–2015) and those who fulfilled the inclusion criteria 227. The DLD children will be followed up during 2014–2018 on a yearly basis or less frequently, depending on whether they are monolinguals or bilinguals and what was their age when entering the study (see, Table  1 ). The last follow-up is before they enter school at the age of seven. The follow-up assessments are conducted mostly in the kindergartens. Children living outside the Helsinki metropolitan area are not followed-up unless they are assessed at Department of Phoniatrics for clinical purposes. Structured questionnaires are used for assessing the content and amount of intervention that takes place during the one-year periods between assessments. Separate questionnaires are sent to kindergartens and speech and language therapists.

In addition, 80 monolingual and 80 bilingual control children are recruited from the kindergartens of the metropolitan area of Helsinki, in order to gather normative information for the neuropsychological and speech and language tests for the sequentially bilingual children, as well as comparison data for the HelSLI subprojects. Control children are gathered from the same areas as DLD children and the proportion of girls versus boys per age group is compatible. The 3-and 4-year-old control children are followed up yearly, until they enter school, in order to define developmental pathways for both monolingual and bilingual TD children. In addition, bilingual 5-year-olds are also followed up until they enter school (see, Table 1 ). At the moment, all the DLD children have entered the study and are being followed up. Also, most of the TD children (over 150 of the total expected n  = 160) have already been recruited to the study. Table 1 presents the general design of the HelSLI study. Below, the methods are described separately for each sub-project.

Temperament is parent-reported with the very short version of The Children’s Behavior Questionnaire (CBQ) [ 55 ]. Parent-child interaction is assessed with structured play sessions that are videotaped in order to evaluate both parenting and child behavior (1990 revision of the Erickson scales, [ 56 ]) and the dyadic level of the parent-child relationship [ 56 , 57 ]. The ways that DLD relates to the psychosocial characteristics and well-being of the children are as assessed with questionnaires Child Behavior Checklist (CBCL) and the Teacher Rating Form (TRF), both part of the Achenbach System of Empirically Based Assessment (ASEBA) [ 19 ] and The Strengths and Difficulties Questionnaire (SDQ) [ 21 ].

Speech and language development is investigated in Finnish, with the same standardized speech and language and neuropsychological test battery in all the groups, that is, monolinguals with typical language development (MonoTD), monolinguals with impaired language development (MonoDLD), and bilinguals with typical (BiTD) and impaired language development (BiDLD; see, Additional file 1 : Appendix 1 and Additional file 2 : Appendix 2). Because of the difficulties in assessing the first language of the bilingual children directly, with or without the help of an interpreter, we implement additionally indirect measures. In the HelSLI-bilingual, these are parent reports on the first language development (The Alberta Language Development Questionnaire, ALDeQ) [ 58 ] and the language environment questionnaire (The Alberta Language Environment Questionnaire, ALEQ) [ 59 ], which have been translated for the present research in collaboration with Professor Johanne Paradis, University of Alberta, Edmonton, Canada.

STM capacities are assessed by asking children to make same/different judgments of small sets of non-linguistic stimuli (pictures or vocalizations of made-up animals), to measure the number of items each child can hold in memory. Nonlinguistic stimuli are used in order to assess memory functions independently from children’s language ability. These tests assess STM for visual and auditory stimuli distributed sequentially. Implicit learning abilities are assessed with AGL tasks [ 52 ] which first show children training examples of small sets of stimuli (similar in nature to those used for the STM tasks), and then ask children to classify novel sets of stimuli as being either “Good” or “Not good” with respect to the presumed pattern exemplified by the training items. These tools were built on the Graphogame literacy training platform ( http://graphogame.com ).

Continuous EEG is recorded during routine clinical checkups at the Department of clinical neurophysiology following clinical standards. Children are sleep deprived and EEG is recorded during a short daytime nap as well as during standard flashlight sequence procedures. During clinical routine EEG assessment, also a tone multifeature MMN paradigm, developed by Näätänen et al. [ 60 ] is used to measure the auditory discrimination profile, which has been shown to be a useful tool for investigating developmental disorders [ 54 , 61 , 62 , 63 ]. The paradigm includes simultaneous measurements for tone frequency, duration, intensity, location, and gap contrasts. Some of the children with DLD and their controls, are invited to participate in more detailed ERP experiments in Cognitive Brain Research Unit, University of Helsinki [ 64 ]. One paradigm allows one to compare basic auditory processing efficiency of different sound features with speech specific sound processing, and thus gives novel insight on the specific neural dysfunctions associated with DLD at the individual level. The second ERP paradigm aims to track the neural circuitry and function needed in morphological processing [ 65 ]. Morphemes are the basic building blocks of the language meaning, and difficulties especially in word inflection have been proposed to be one of the core problems in DLD. This novel paradigm will now be used in children for the first time. Together all of these ERP paradigms allow specifying neurophysiological indices associated with cognitive dysfunction in DLD at the levels of basic auditory processing, phonological processing, and STM as well as morphological processing. This multilevel approach is particularly important as it allows the development of more reliable individual level indices and their comparison with cognitive and genetic measures of the HelSLI.

DNA in the HelSLI-genetic is extracted from saliva and analyzed by the international collaborators. Two sets of DNA markers are assayed. The first is a set of single nucleotide polymorphism (SNP) markers that constitute a DNA “barcode” that are unique across the population and are used for sample tracking and to assess relatedness among individuals [ 66 ]. That same set of SNPs was chosen to be ancestrally informative to provide information on continental genetic background to statistically control for admixture [ 66 ] across the control and DLD groups. A second set of SNP markers will provide information about common variation in the four (known) DLD genes. Analysis consists of methods previously deployed on similar datasets [ 67 ]. Briefly, ancestrally informative markers are analyzed by principal component analysis to provide a genomic summary of ancestry. We have shown that it is important to use the first three principal components as a covariate to reduce false positive associations across groups caused by random differences in ancestry [ 66 ]. The main genomic effects are modeled along with other variables in the regression framework using dummy coding to represent each of the three genotypic groups (AA, AB, BB; where A generically refers to the common SNP variant, and B generically refers to the more rare variant of the two).

Characteristics of participants

The HelSLI study recruited four groups, that is, monolingual DLD (MonoDLD), bilingual DLD (BiDLD), monolingual TD (MonoTD), and bilingual TD children (BiTD). DLD children came from the Audiophoniatric Ward for children, Department of Phoniatrics. The TD children were gathered from kindergartens around the greater Helsinki area. In general, all four groups participate in all the subprojects of HelSLI, that is, psychosocial, bilingual, cognitive, EEG, and genetic (for exceptions, see Table 1 ).

Inclusion criterion for the DLD children was a referral to the Audiophoniatric Ward, Department of Phoniatrics, with a continuing concern in language development (in bilinguals in both languages) with no known biomedical etiology [ 68 ] (see Table  2 for sample description). Parent interviews and/or language assessment with the help of interpreter on first language (L1) had to confirm severe challenges in child’s first language. The children had a prior SLT assessment/intervention period in primary health care. They had normal hearing and no gross neurological findings, and had participated in routine follow-ups in local health-centers. In the ward, a medical examination, including ear-nose-and-throat (ENT) areas, gross and fine motor skills roughly, and a brief gross neurological status to rule out major findings or signs of any syndrome, was performed.

In most cases, the DLD children are analyzed as one group, that is, we do not differentiate between, for example, receptive and receptive-expressive groups. However within the DLD children, a group with severe speech production problems on phonology/speech sound level is separated, since severe disorder in speech production may affect speech intelligibility and by implication expressive language (e.g. expressive vocabulary and sentence production). This distinction was necessary to make because in the Finnish ICD-10 [ 69 ] system speech sound disorders (such as CAS, childhood apraxia of speech) are included in SLI or DLD (ICD-10 diagnosis of F80.1). Classification for children with or without severe speech production problem based on difficulties at the phonological or speech sound level was made by combining the results from Finnish test of phonology (Fonologiatesti) [ 70 ] and speech and language therapist’s clinical report. In the Phonology test, the child had to perform below 12. percentile on phonotactic skills and in relation to age she/he had to have a significantly small phoneme inventory and/or severe difficulties in combining phonemes. If inclusion to the speech production problem group was made based on small phoneme inventory, omitted or substituted phonemes needed to be more than two and they had to be other than late emerging phonemes /r/ and /s/ or phonemes used only in loanwords. Children who did not produce speech at all were considered as their own group in some analyses.

Exclusion criteria for the DLD group were hearing impairment, intellectual disability, ASD, oral anomalies, or a diagnosed neurological impairment or disability (e.g., epilepsy, chromosomal abnormalities). The DLD children were required to have a performance Intelligence quotient (PIQ) of at least 70 [ 71 ]. For research purposes, the DLD group was divided into those who had PIQ in the range 70–84 and 85 or above. However, we did not require a mismatch between the verbal and nonverbal ability and we acknowledged the fact that DLD can co-occur with other neurodevelopmental disorders [ 68 ].

The TD children were gathered from kindergartens around the greater Helsinki area. They were required to not have difficulties in any of their languages or no intervention after an assessment. Guidance or short intervention period focusing on articulation, i.e. individual speech sounds, were not considered as exclusion criteria. The parents of TD children were required not to report any of the exclusion criteria and the TD children were required to have PIQ of at least 85 [ 71 ]. Further, exclusion criteria for the TD children were suspected or diagnosed difficulties in language acquisition or other development as well as diagnosed difficulties in these areas in parents or siblings.

Monolingual participants were required to have Finnish as their only home language. Sequential bilingual children vary in their first language (L1), but were required to have only one language at home (not Finnish, Swedish, or Sami). L1 languages in bilingual TD children were compatible to the ones of DLD children. Bilingual children had to have had at least one year of regular exposure to Finnish language in kindergarten. There are no standardized tests nor normative info on sequential bilingual performance in Finnish language-related tests. Therefore, we could not establish clear cut-off criteria for the test performance of the participating groups.

Statistical analyses

A priori power analyses with G*Power [ 72 ] and RMASS (http://www.rmass.org/) were conducted to estimate appropriate sample sizes. For various research questions of subprojects guesstimates for the effect size varied along with the other aspects of power analysis. Detailed descriptions go beyond the scope of this paper, but two examples are given. For one age group (that is, e.g., 3 years old) an effect size as Cohen’s d  = 0.6 was used for independent samples two-tailed t-test between DLD and TD children with α = .05 and 1 - β = .80 (power) using sample ratio N DLD / N TD  = 0.67. This calculation resulted in N DLD  = 56 and N TD  = 38 for each age. The total number of participants recruited approximates these values (227 with suspected DLD, plus 160 TD across the four age groups). As another example, we computed the sample size for two-level mixed-effects linear regression model for the analysis of longitudinal data using the aforementioned values for α, 1 - β and sample ratio, four time points with AR1 error variance = 1.0 and r  = .5, last time point mean difference = 0.6, 5% attrition rate, person variance components (intercept = 1.0, covariance = 0.1, slope = 0.1), and group × time interaction = 0.2. Here total number of subjects was 353. Again, the number of participants recruited (227 + 160 = 387) approximates the number indicated by the power analysis.

With large dataset and different subprojects, several different analytical lines will be pursued contingent upon the particular research questions of each subproject. Subsequent publications will describe details of the analysis used in each of them and only general tactics will be illustrated here. When all t 0 (onset, baseline) assessments are finished, cross-sectional analyses will be carried out to explore relationships between variables of interest in each subproject. These analyses will include, e.g., different general linear modelling, multivariate analysis, and structural equation modelling techniques. In specific research questions, also generalized modelling may be used. As t 1 , t 2 , and t 3 (follow-up) data is complete, longitudinal analysis (especially pertinent in HelSLI-bilingual) will be conducted. For this, multilevel modelling techniques for longitudinal data will be applied.

Both frequentist and Bayesian approaches to inference will be utilized depending on research questions of each subproject. In the former case, two-tailed nominal p -value of .05 and 95% confidence interval and, in the latter case, informative priors, when realizable, and 95% credible interval will be generally used.

Following the multiple deficit model of developmental disorders put forward by Pennington [ 73 ], the HelSLI subprojects investigate the DLD phenomenon at multiple levels of analysis: genetic and environmental etiological, neural, cognitive, behavioral, and psychosocial (see Fig.  1 ). The main aim of the project is to increase our understanding of the multiple interactive risk and protective factors that affect the developing heterogeneous cognitive and behavioral profile of DLD. Data collection is in active stage and the collected data will be unique in the world in its quality and quantity.

Levels of the study and description of HelSLI subprojects

At the level of etiological risk and protective factors (see Fig. 1 ), we will be able to investigate the associations between biology (genes, temperament) and environment (parent-child interaction and language background) and use this knowledge, for example, to predict intervention outcomes and as covariates at other levels of analysis. At the level of neural systems, we will be able to investigate the neurophysiological correlates of DLD (both continuous EEG characteristics and ERP responses to various linguistic and non-linguistic auditory stimuli), evaluate the usefulness of EEG/ERP in individual diagnostics, and map these findings to the etiological level of analysis. We can determine, for example, the associations between genetic and language background and brain electrophysiology.

At the level of cognitive processes, we will be able to investigate the difficulties in nonlinguistic basic cognitive capacities that are expected to affect DLD across different languages with the aim to use this knowledge to develop language-independent tools for prediction, diagnosis, and intervention of DLD and later dyslexia. As described in the Background section, genetic factors behind DLD appear to affect not only language but also nonverbal performance. Especially (nonlinguistic) STM and procedural learning will be of interest here, since these have been associated also with the etiological and neural levels of analysis. At the level of behavioral manifestation, we will be able to investigate the variation ranging from typical to severely impaired language development. This level of analysis will enable testing for and validating subgroups suggested by the other levels of analysis (e.g., EEG abnormalities emerging in those with comorbid difficulties). Last, at the level of psychosocial outcome, we will be able to investigate associations between the other levels and a child’s psychosocial characteristics and well-being. With all these levels of analysis, the HelSLI study will be in a unique position to define correlative and probabilistic or derivational causal relations and map developmental pathways (or trajectories) in a large longitudinal sample. Moreover, there is little previous research into the relationship between bilingualism and DLD, and none that spans all these levels of analysis.

As the project will be carried out in a clinical setting, traditional and experimental assessment and intervention methods can be employed as part of the research project, in order to provide the DLD children comprehensive services. This and the longitudinal design make it possible to distinguish between associated and causal factors. The results could be used to help predict language development and its difficulties across language environments. Based on the results of the assessments, the current project will provide means for targeting some of the possibly causative factors, not just the resulting symptoms, with, for example, the adaptive computerized interventions of HelSLI cognitive that can be individually tailored based on the differences at the etiological, cognitive, and behavioral levels of analysis. This kind of early intervention in the promotion of health and equality and prevention of marginalization is pivotal, since funding targeted at supporting learning during the early years of education results in better outcomes than that provided during the later years [ 74 ].

Abbreviations

Attention deficit/hyperactivity disorder

• Artificial grammar learning

Alberta Language Development Questionnaire

Alberta Language Environment Questionnaire

Autism spectrum disorders

Achenbach System of Empirically Based Assessment

Childhood apraxia of speech

Child behavior checklist

Children’s behavior questionnaire

• Developmental language disorder

Electroencephalography

ear-nose-and-throat

Event-related potential

Helsinki Longitudinal SLI study

Health-related quality of life

First language

Second language

Mismatch negativity

Monolingual

Neuropsychological

Performance Intelligence quotient

Strengths and difficulties questionnaire

• Specific language impairment

Speech and language therapy

Single nucleotide polymorphism

Short term memory

Typically developing

Teacher rating form

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Acknowledgements

Authors of the current article participated in all or specific subprojects of HelSLI

All subprojects

Marja Laasonen, Principal investigator of HelSLI

Sini Smolander

Eva Arkkila

For each subproject below, the participants are in alphabetical order

HelSLI-psychosocial:

Kati Heinonen

Anu-Katriina Pesonen

HelSLI-bilingual:

Sari Kunnari

HelSLI-cognitive:

Todd M. Bailey

Pekka Lahti-Nuuttila

Emmanuel M. Pothos

Elisabet Service

HelSLI-EEG:

Teija Kujala

Hanna-Reetta Lajunen

Leena Lauronen

Miika Leminen

Paavo H. T. Leppänen

HelSLI-genetic:

Christopher W. Bartlett

Ahmed Geneid

We thank the following persons for their invaluable contribution to the specific subprojects.

Professor Dorothy Bishop, University of Oxford, UK (HelSLI-cognitive)

Professor Heikki Lyytinen, University of Jyväskylä, Finland (HelSLI-cognitive)

Professor Johanne Paradis, University of Alberta, Canada (HelSLI-bilingual)

Iida Porokuokka, University of Jyväskylä, Finland (HelSLI-cognitive)

MD, PhD Erkki Vilkman, previous Head of the Department of Phoniatrics, Helsinki University Hospital

All SLTs, psychologists, phoniatricians, nurses, and other personnel at the Department of Phoniatrics, University of Helsinki and Helsinki University Hospital

Numerous research assistants contributing to the data gathering

All participating children and their families as well as kindergartens and their personnel

Helsinki Uusimaa Hospital District funding covers the clinical part of the project and additional research funding. Also, the Academy of Finland funds the project. These two sources of funding cover the additional costs of data gathering for the DLD children as well as salaries of the research group, that is, design of the study and collection, analysis, and interpretation of data and writing the manuscripts. The Social Insurance Institution of Finland (Kela) funds the project with two grants, which cover the assessments of the TD children.

Availability of data and materials

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Author information

Authors and affiliations.

Department of Otorhinolaryngology and Phoniatrics, Head and Neck Surgery, Helsinki University Hospital and University of Helsinki, Haartmaninkatu 4 E, 00029 HUS, POB 220, Helsinki, Finland

Marja Laasonen, Sini Smolander, Pekka Lahti-Nuuttila, Miika Leminen, Ahmed Geneid & Eva Arkkila

Department of Psychology and Logopedics, University of Helsinki, Helsinki, Finland

Marja Laasonen, Pekka Lahti-Nuuttila, Kati Heinonen & Anu-Katriina Pesonen

Department of Psychology and Speech-Language Pathology, University of Turku, Turku, Finland

Marja Laasonen

Research Unit of Logopedics, University of Oulu, Oulu, Finland

Sini Smolander & Sari Kunnari

School of Psychology, Cardiff University, Cardiff, UK

Department of Psychology, City University of London, London, UK

Centre for Advanced Research in Experimental and Applied Linguistics, Department of Linguistics and Languages, McMaster University, Hamilton, Canada

Cognitive Brain Research Unit, Department of Psychology and Logopedics, University of Helsinki, Helsinki, Finland

Miika Leminen & Teija Kujala

Department of Clinical Neurophysiology, Hospital for Children and Adolescents, University of Helsinki and Helsinki University Hospital, Helsinki, Finland

HUS Medical Imaging Center, Clinical Neurophysiology, Helsinki University Hospital and University of Helsinki, Helsinki, Finland

Department of Psychology, University of Jyväskylä, Jyväskylä, Finland

Battelle Center for Mathematical Medicine, The Research Institute at Nationwide Children’s Hospital & The Ohio State University, Columbus, USA

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All named authors participated in the design of the study as well as in manuscript preparation. For this manuscript and for the study in general, rights and responsibilities of the participating students and researchers, requirements for authorship as well as the rights of ownership and use to the data are defined in written contracts for each separate subproject. Authorship decisions are made based on the Defining the Role of Authors and Contributors guidelines ( http://www.icmje.org/ ). All authors read and approved the final manuscript.

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Appendix 1 Neuropsychological assessment battery. List of neuropsychological assessments used in the study. (DOCX 81 kb)

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Appendix 2 SLT assessment battery. List of speech and language assessments used in the study. (DOCX 25 kb)

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Laasonen, M., Smolander, S., Lahti-Nuuttila, P. et al. Understanding developmental language disorder - the Helsinki longitudinal SLI study (HelSLI): a study protocol. BMC Psychol 6 , 24 (2018). https://doi.org/10.1186/s40359-018-0222-7

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Specific Language Impairment, Autism Spectrum Disorders and Social (Pragmatic) Communication Disorders: Is There Overlap in Language Deficits? A Review

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Analysing language characteristics and understanding their dynamics is the key for a successful intervention by speech and language therapists (SLT). Thus, this review aims to investigate a possible overlap in language development shared by autism spectrum disorders (ASD), specific language impairment (SLI) and social (pragmatic) communication disorder (SPCD). The sources of this work were the PubMed, PsycInfo and SciELO databases, as well as the Scientific Open Access Repositories of Portugal. The final selection included 18 studies, focused on several linguistic areas. Results suggest that when individuals are matched according to some language or cognitive skills, they will also show similar characteristics in other language domains. Future work should be done based on spontaneous speech.

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Introduction

Clinical typologies of developmental language disorders are based either on etiological criteria or symptomatic criteria. Both types have shortcomings and limitations. Frequently, typologies based on etiological factors cannot explain properly the variable symptomatology exhibited by patients. Typologies based on symptomatic factors often fail to categorise and characterise patients unambiguously, essentially because of the widespread problem of comorbidity. Comorbidity entails that patients can exhibit symptoms that are characteristic of two or more different disorders, this resulting in a partial overlap of clinical categories. In truth, this overlap can be observed also at the brain level (with damages in the same region resulting in different disorders) and the genetic level (with mutations in the same gene resulting in diverse conditions). These circumstances make an accurate diagnosis of disorders troublesome, as there seems not to exist a causal link between specific language problems and specific cognitive deficits (or brain areas or gene mutations). Figure  1 summarises this complex scenario. Ultimately, this has a negative impact on the therapeutic approaches aimed to improve the language disabilities of people with these conditions. This problem is not easy to fix. As discussed by Benítez-Burraco ( 2020 ), for fixing this, it is urgent to consider developmental dynamics, from genes to language deficits. In order to understand the real nature of disorders, one needs to pay attention not only to the symptoms observed in the adult state, but also to how disorders manifest throughout development (Benítez-Burraco, 2013 ). Overall, this holistic approach should help find reliable endophenotypes of language disorders, that is, disorder-specific biological markers for each condition.

An indirect link between cognitive and language problems in developmental disorders impacting on language. A The links between cognitive deficits and language problems in developmental language disorders are not straight or univocal. B The links between language problems and aspects of the grammar are not straight either and change thorough development

This review explores these issues, focusing on three disorders impacting on language that are defined symptomatically and that are often correlated by researchers: autism spectrum disorders (ASD), specific language impairment (SLI) and social (pragmatic) communication disorder (SPCD). The ultimate aim is to investigate whether a distinctive symptomatic profile can be proposed for each disorder in the domain of language to enable an accurate diagnosis, in spite of some overlap between symptoms. The paper is structured as follows: we first provide a detailed characterisation of the language deficits observed in children with these three conditions, as it can be found in the relevant literature, with a special focus on the structural aspects of language (syntax, semantics), but also on how language is put into use (pragmatics). We then conduct a meta-analysis of the papers, by examining the simultaneous presentation of these disorders in patients, with the aim of fully identifying the overlapping aspects of the phenotype and the domains in which one can find disorder-specific features. We finally provide a detailed discussion of our results, particularly, how our analysis can contribute to achieve a better understanding of language deficits, strengths and dynamics in children with SLI (or DLD), ASD and SPCD, and ultimately, to achieve better therapeutic interventions.

Autism Spectrum Disorder

According to the classification established by DSM-5 (APA, 2013 ), ASD is defined by the presence of persistent deficits in social communication and social interaction across multiple contexts and restricted/repetitive patterns of behaviour, interests or activities. This disorder aetiology has been extensively explored in recent years, and may be related to different factors like genetics, advanced paternal or maternal age (Croen et al., 2007 ) and the grandparents age at parents’ birth (Gao et al., 2020 ), among others. ASD symptoms must be present in the early developmental period and cause clinically significant impairment in social and occupational areas, or other dimensions of individual current functioning. Autism severity is defined by three levels, based on ‘social communication’ problems and ‘restricted, repetitive behaviours/interests’ (RRBIs) (APA, 2013 ).

Currently, there are standard diagnostic instruments that afford a diagnosis of ASD, such as the Screening Tool for Autism in Toddlers and Young Children (STAT) or the Autism Diagnostic Observation Schedule (ADOS), which can be applied from 1 year old (Lord et al., 2018 ). However, some researchers suggest that detection and diagnosis of ASD can only start accurately from 14 months of age (Pierce et al., 2019 ), through the administration of multiple direct assessment and parent-report instruments (Nevill et al., 2019 ). Regarding language assessment in high-functioning autism, Loukusa et al. ( 2018 ) suggest that the use of context-sensitive materials, like the Pragma test (Loukusa et al., 2018 ), allows the detection of social-pragmatic inferencing difficulties not only in real-life situations but also through parental reports, and in structured test situations.

Although little has been studied about language development in ASD during the first years of life, it is known that the development of expressive language is slower compared to language comprehension (Bruyneel et al., 2019 ), and it is also known that gestures, non-verbal cognitive ability and response to joint attention are significant predictors of receptive language. On the other hand, non-verbal cognitive ability, gestures and imitation are the most important predicting factors of expressive language skills (Luyster et al., 2008 ).

ASD may have concomitant language impairment (ASD-language impairment (LI)) or not (ASD-no language disorder (NLD)). Autistic children with LI showed lack of neural functional differentiation to speech stimuli in the superior temporal cortex and, similarly, a much lower activation pattern related to general auditory processing, compared to typically developed and autistic NLD peers. This suggests that abnormalities in auditory processing could be specifically related to a dysfunction in language and speech neural systems (Lombardo et al., 2015 ) and may impair the ability to process the speech of other people, hence reducing the ability to learn the phonology, syntax and semantics of one’s native language (Yau et al., 2016 ). Moreover, autistic individuals present a decrease in cortical thickness in cerebral areas related to pragmatic language and social communication abilities (Crutcher et al., 2018 ).

In relation to gender differences, it is known that autistic females show a higher performance in narrative production, including more visible history elements and more descriptors of planning and intention (Conlon et al., 2019 ).

ASD and Pragmatics

Pragmatics is defined as the study of language in relation to context (Hart, 1981 ). Since there is a wide range of autistic individuals, being that around 25 to 30% of autistic children either fail to develop functional language or are minimally verbal (Brignell et al., 2018 ), La Valle et al. ( 2020 ) compared the pragmatic speech profiles in minimally verbal and verbally fluent autistic individuals, aged between 6 and 21 years old, through natural language sampling. This study firstly concludes that minimally verbal autistic individuals were not restricted to one communicative function. The primary communicative function used by this group was indicating agreement/acknowledgement/disagreement, followed by other communicative functions (such as responding to a question, requesting and labelling or naming an item/thing). The second conclusion was the use of comments as the key function distinguishing the two groups. Thus, for verbally fluent autistic individuals, the primary pragmatic function used is commenting, followed by responding to a question, and indicating agreement/acknowledgement/disagreement/refusal.

Pragmatic skills are often related to the theory of mind (ToM), that is, to the ability to attribute mental states to oneself and others (Cole & Millett, 2019 ). Explicit ToM skills begin to develop at 4 years old, with the understanding of false beliefs (Poulin-Dubois et al., 2020 ), and continue to evolve across the adult lifespan, with age being associated with decline in both cognitive and affective ToM skills (Laillier et al., 2019 ). Assessment of ToM skills requires the use of reliable measures and scales. However, although ToM has other important aspects (diverse desires; diverse beliefs; knowledge access; hidden emoticons; and understanding irony and idioms), the false belief tasks are the most implemented (Smogorzewska et al., 2018 ). Smogorzewska et al. ( 2018 ) consider that application of both The Theory of Mind scale (Wellman & Liu, 2004 ) and the Faux Pas Recognition Test (Baron-Cohen et al., 1999 ) assessment measures cover different aspects of ToM development and allow for a complete assessment. These measures also have the advantage of validation in children with and without disabilities. In adulthood, Laillier et al. ( 2019 ) suggest the administration of the Movie for Assessment of Social Cognition (Dziobek et al., 2006 ) to assess both cognitive and affective ToM skills.

Comparison Between Pragmatic Skills of Autistic Children and Typically Developed (TD) Peers

At the pragmatic level, as well as in social adaptation, ToM and executive functions, there are significant differences between autistic children and typically developed (TD) peers (Berenguer et al., 2018 ; Garrido et al., 2017 ). Autistic children have difficulties in understanding emotional speech, so they have problems drawing appropriate inferences, especially in multiple-cue environments (Le Sourn-Bissaoui et al., 2013 ), and present deficits in appreciating irony and sarcasm (Solomon et al., 2011 ).

Whyte and Nelson ( 2015 ) investigated difficulties in understanding pragmatic language and nonliteral language in children between 5 and 12 years old. The authors verified that, although these competences increase significantly with chronological age, autistic children showed slower rates of development, when compared to TD peers. They also concluded that pragmatic language and nonliteral language skills are at the same level as vocabulary comprehension and ToM competences in autistic children. Therefore, ToM skills, as structural language (speech, syntax, semantics, coherence) and working memory, are significant predictors of the pragmatic skills in autistic individuals (Baixauli-Fortea et al., 2019 ; Schuh et al., 2016 ). Regarding autistic individuals, pauses during discourse can be filled with words like ‘um’ or ‘uh’, being that ‘um’ is used to signal longer pauses and may correlate with greater social communicative sophistication than ‘uh’ (Parish-Morris et al., 2017 ). The ‘um’ rate is associated with autism symptom severity (Irvine et al., 2016 ). The use of these fillers can thus contribute to distinguish autistic individuals from their TD peers. However, since girls use ‘uh’ less often than boys, their language impairment can be more easily camouflaged (Parish-Morris et al., 2017 ).

ASD and Semantics

Linguistic semantics is the study of literal meanings grammaticalised or encoded (Frawley, 1992 ). In general, autistic individuals present difficulties in high-level functions, like semantic integration (Coderre et al., 2017 ), due to presenting a delayed rate of processing and limited integration of mental representations (DiStefano et al., 2019 ). Receptive semantic knowledge of this population is sensitive to context conditions—semantic comprehension improves, when contextualised (Lucas et al., 2017 ). Thus, increasing the richness and complexity of semantic contexts helps autistic children to learn new words over time (Gladfelter & Goffman, 2018 ). Even so, autistic children with LI need additional stimuli, such as explicit teaching of words or additional tuition for learning, compared to autistic NLD children (Lucas et al., 2017 ).

Comparison Between Semantic Skills of Autistic Children and TD Peers

The most pronounced difference between autistic individuals with LI and their TD peers is at semantic-pragmatic level (Westerveld & Roberts, 2017 ). Individuals with SLI-LI show difficulties in naming and understanding compound words, even knowing each word included in a compound (Kambanaros et al., 2019 ). Verbal pre-schoolers on the autism spectrum evidence specific difficulties in oral narrative comprehension and production skills (intelligibility and grammatical accuracy), producing simple narratives that lack semantic richness and omit important story elements (Norbury et al., 2014 ). Their speech is characterised by low levels of language abstraction, with few words related to feelings (Chojnicka & Wawer, 2020 ) and production of descriptive or action sequences (Westerveld & Roberts, 2017 ), with reduced references to semantic-pragmatic elements, as basic story details (e.g. characters, settings, actions) and complex concepts, reflected in the story’s central ideas (Kenan et al., 2019 ). Later, in school age, the narratives of autistic children remain syntactically less complex, contain more ambiguous pronouns and include fewer story grammar elements than those of TD peers (Banney et al., 2015 ). Losh and Gordon ( 2014 ) verified that narrative competence between 8 and 14 years old is comparable to controls in terms of semantic content when narrating from a picture book. However, narrative recall tasks remain a major challenge for autistic individuals, showing poor semantic content.

ASD and Syntax

In linguistics, syntax refers to the organisation of words in sentences and morphology refers to the internal structure of words (Sim Sim, 1998 ). Autistic individuals, even with high-functioning ASD (HFA), show syntactic and morphological impairments that should not be overlooked (Brynskov et al., 2017 ). They (especially ASD-LI) perform significantly worse than controls in relative clause comprehension (Durrleman et al., 2015 ; Garrido et al., 2017 ) . The use of complex syntax is also a weak area, since they have difficulties integrating their narratives and explaining characters’ intentions (Lee et al., 2018 ). Autistic individuals rarely produce clitics on their utterances (Terzi et al., 2016 ), ‘wh’ questions (Goodwin et al., 2012 ) and mental verbs (Song et al., 2017 ). However, these problems seem to be related to the interface between (morpho)syntax and pragmatics. Since, generally, autistic individuals have specific topics of interest, if the conversation is not revolving around those specific topics, they may not get enough reinforcement to carry on with the conversation. Thus, autistic children produce mainly noun phrases, instead of using clitics, indicating that they do not know that a clitic should be used to refer to a prominent entity in the preceding discourse (Terzi et al., 2016 ). In relation to ‘wh’ questions, it is known that even when they are understood by autistic individuals around 4½ years old, they are rarely produced. This can be due to several factors. One possible explanation is related to pragmatic weakness, as these individuals have difficulties in deliberate seeking of new information, assuming that such information is known by one’s addressee. Still, autistic individuals present limitations knowing when and how such questions fit into discourse (Goodwin et al., 2012 ). On the other hand, it may be due to motivational issues, such as social motivation or IQ influence of expressive language (Kim et al., 2020 ).

Specific Language Impairment

SLI is characterised by language difficulties that do not arise from any known neurological, sensory or emotional causes (Ervin, 2001 ). The terminology used in this context has been the subject of much discussion since the term ‘specific’ does not apply to the majority of cases with this diagnosis. Still, the designation developmental language disorder (DLD) included in the International Classification of Diseases–11 th Revision (ICD-11) has a very similar definition to that of SLI: these disorders are caused by persistent language acquisition, comprehension, production or use deficits, which arise during development and impair the subject’s communicative competence. Language skills are thus remarkably below expectations, considering the age and level of intellectual functioning, and this is not caused by any other neurodevelopmental disorder, due to sensory deficit or neurological condition (ICD-10, 2019). In recent years, several other terms have been used to designate these impairments, with no agreement or consensus between authors. The classification ‘language impairment’ used in the DSM-5 also becomes problematic because it covers a wide range of disorders (Bishop, 2014 ). This literature review follows Bishop’s ( 2014 ) line of reasoning, conceiving the term ‘specific’ as ‘idiopathic’ (i.e. of unknown origin), rather than implying there are no other disorders beyond language. Therefore, we decided to maintain the term ‘specific language impairment’. We also use the term ‘developmental language disorder’ (ICD-11), since we consider that both designate the same type of disorder (Gladfelter et al., 2019 ).

SLI (or DLD) and Pragmatics

Pragmatic disorders in individuals with SLI are seen differently by several authors. While some say that individuals with SLI are distinguished from autistic individuals or individuals with SPCD by the absence of social impairment (e.g. Gibson et al., 2013 ), others say that the screening of pragmatic skills while evaluating the communication skills of individuals with SLI should be seriously considered (Osman et al., 2011 ). In fact, there are some pragmatic skills that are affected in individuals with SLI (or DLD), as the maxim of quantity in sentence answers to ‘wh’ questions (Rombough & Thornton, 2018 ). Also, Katsos et al. ( 2011 ) confirmed that children with SLI do face difficulties in employing the maxim of informativeness as well as in understanding the logical meaning of quantifiers, and that these difficulties accompany their overall language difficulties. During narrative production, there are several impairing aspects, such as referencing, event content, mental state expressions and inferencing (Mäkinen et al., 2014 ). Even so, there is a wide range of individuals with SLI with different pragmatic skills that affect their relationships with peers. Overall, individuals with better pragmatic language skills and lower levels of emotional problems have less difficulty in developing peer relations (Mok et al., 2014 ).

SLI and Semantics

The neuronal processing of semantic information at sentence level is atypical in pre-schoolers with SLI (Pijnacker et al., 2017 ), and this reflects in their speech production, with longer silent pauses than TD individuals (Befi-Lopes et al., 2013 ). Children with SLI reveal deficits in lexical-semantic organisation, showing difficulties in lexical access (Girbau, 2014 ; Sheng & McGregor, 2010a , b ). They have word-learning difficulties, potentially originated in the early stages of the process of fast mapping (Jackson et al., 2016 ) and statistical learning. The latter is related to lexical-phonological abilities, predicting them (Mainela-Arnold et al., 2010 ). Therefore, the ability of individuals with SLI to learn novel words increases when stimuli combine visual and verbal information (Gladfelter et al., 2019 ). However, it is possible to discriminate among children with SLI those who present greater lexical deficits (Befi-Lopes et al., 2010 ), showing associations between vocabulary level and naming abilities (Sheng & McGregor, 2010a , b ) or lexical retrieval (Novogrodsky & Kreiser, 2015 ). Generally, all individuals with SLI perform worse when naming verbs compared to objects, which reveals problems encoding semantic representations (Andreu et al., 2012 ). This deficit is specific to the verbal domain, suggesting weakened and/or less efficient connections within the language networks (Cummings & Ceponiene, 2010 ).

SLI and Syntax

SLI has been traditionally characterised as a deficit of structural language (specifically grammar) (Davies et al., 2016 ). Children with SLI frequently omit tense-related morphemes (Rombough & Thornton, 2018 ) and present weaknesses in noun and verb inflections at 5 years old. At pre-school age, number agreement is a major challenge for children with SLI, with difficulties especially in oral production of ‘quantifier + noun’, compared to ‘determiner + noun’ (Rice & Oetting, 1993 ). However, all these difficulties tend to disappear with age. At 8 years old, individuals with SLI produce fewer relative clauses (Zwitserlood et al., 2015 ) and complex sentences (Domsch et al., 2012 ) than their TD peers. For children with SLI, some kinds of relative clauses are easier than others, displaying a similar profile to TD children but at a lower level of performance (Frizelle & Fletcher, 2014 ). At the age of 10, only complex sentence structure generation remains difficult (Ingram, 2019 ).

It is also important to highlight that some difficulties manifested by individuals with SLI, like the use of clitics (Stanford et al., 2019 ), as well as understanding complex sentences that include non-finite subject-verb sequences (Souto et al., 2016 ), are probably related to memory skills, rather than limitations on syntax (Montgomery et al., 2016 ). Individuals with SLI who show weaknesses in tense marking and verb agreement also reveal difficulties in non-word repetition, that is, in phonological short-term memory (Ebbels et al., 2012 ).

Social (Pragmatic) Communication Disorder

SPCD constitutes a recently created diagnosis (DSM-5, 2013), previously included in the ASD diagnosis, being the main difference the severity level of behaviours (Reisinger et al., 2011 ). SPCD refers to persistent difficulties in social use of verbal and non-verbal communication, deficits in understanding and following social rules of communication in natural contexts and difficulties in adjusting communication to match context or the needs of the listener and in following storytelling and rules of conversation (APA; DSM-5, 2013). However, there is a lack of reliable and culturally valid assessment measures to make a differential diagnosis of SPCD (Norbury, 2014 ), and there is no qualitative evidence to distinguish SPCD from ASD. SPCD appears to lay on the borderline of the autism spectrum, describing those with autistic traits that are insufficiently severe for ASD diagnosis, but who nevertheless require support in the field of pragmatics (Mandy et al., 2017 ). Gibson et al. ( 2013 ) suggest that a rigorously defined diagnostic group of individuals with SPCD can be differentiated from those with HFA, with the support of a more detailed measure of RRBIs, focused on current functioning in everyday contexts. Lockton et al.’ ( 2016 ) research reveals that some children with SPCD show awareness of the pragmatic rules they themselves do not follow when conversing, so it may be beneficial for therapeutic intervention to focus on improving motivation for use and better understanding of the impact of one’s own pragmatic performance on others.

Between 5 and 7 years old, children with SPCD show difficulties in narrative competence, especially in narrative productivity and story content organisation. Although their developmental trajectory is largely similar to that of TD individuals, a persistent developmental delay of approximately 1 year is observable (Ketelaars et al., 2016 ).

Common aspects of language development in individuals with SLI and autistic individuals have been subject of research in recent years. However, despite the similarities observed regarding social interaction (Swineford et al., 2014), there are few studies including individuals with SPCD.

Bearing in mind that (1) in individuals with SLI, as well as in autistic individuals with LI, changes in comprehension as well as alterations in the use of language and in the structural component of language can be verified; and (2) in individuals with SPCD, only difficulties in terms of use of language are expected, this work aims to investigate if the dissociation between the three pathologies is factual or not.

This raises a central question in speech-language pathology: is there evidence to identify developmental language trajectories across autistic individuals and individuals with SLI/DLD and SPCD?

This literature review used the main research databases in this field: PubMed, PsycInfo and SciELO. We have also researched ‘scientific grey literature’ (not published literature) through the Scientific Open Access Repositories of Portugal. We used the keywords ‘Autism Spectrum Disorders’ and/or ‘Social (Pragmatic) Communication Disorder’ and/or ‘Spec i fic Language Impairment’ or ‘Developmental Language Disorder’. We selected papers published in the last 10 years in Portuguese, English, Spanish and French, languages spoken by the authors of this review, which focused on autistic individuals and individuals with ASD, SLI/DLD and SPCD. The keywords ‘Autism Spectrum Disorders’, ‘Spec i fic Language Impairment’, ‘Developmental Language Disorder’ and ‘Social (Pragmatic) Language Impairment’ were used. The articles were reviewed and selected with full consensus among all authors, according to the following inclusion criteria: (1) they always included simultaneous language assessments regarding at least two of the studied populations; (2) comparative studies of any research design. The following exclusion criteria were defined: (1) assessment of only one of the target groups; (2) non-inclusion of the analysis of any aspect related to the understanding and/or expression of oral language. The research conducted with these databases identified 326 articles: PubMed, 301; PsycInfo, 7; SciELO, 5; Scientific Open Access Repositories of Portugal, 13. After eliminating the repeated papers and performing the first selection, according to the inclusion and exclusion criteria, by examining title and abstract, 18 papers were chosen for full text analysis. Figure  2 shows the selection criteria.

Selection criteria of the articles

The 18 included papers contemplate simultaneous language assessments with the study populations, comparing their results. In total, 1496 individuals, aged between 1 and 15 years old, were assessed. Of these participants, 482 were considered to have typical development (control group), 590 were diagnosed with ASD, 259 with SLI or DLD and 22 with SPCD.

Table 1 summarises the main contents of the included studies. Most of the authors divided the participants with ASD in two groups: ASD-LI and ASD-NLD. Phonological working memory (PWM), measured by pseudoword repetition, is the most investigated area.

Phonological Working Memory

Five research papers were included with comparable methodologies in the area of PWM. Of these studies, only Tager-Flusberg’s ( 2015 ) identified a similar performance pattern among autistic individuals with LI and individuals with SLI, in terms of phonological errors, produced in pseudoword repetition. In the others, participants with SLI always displayed a weaker performance.

Semantic Skills

Semantic skills are addressed in four studies with distinct objectives and methodologies. The authors verified that children with SLI and autistic children with LI have similar lexical deficits regarding semantic knowledge, associations between lexicon and syntax through vocabulary, word definition/matching and sentence recalling tests (from a given word) and have difficulties in understanding compound nouns and interpreting the first noun as the agent. Haebig et al. ( 2015 ) also conclude that lexical-semantic knowledge is similarly organised in school-age autistic children and children with SLI, matched on receptive vocabulary knowledge. This study also suggests that lexical-semantic knowledge in autistic individuals and individuals with SLI may be immature but follows a similar organisation of knowledge as TD individuals. The mechanisms underlying word learning (statistical learning and fast mapping) were investigated by Haebig et al. ( 2017 ) through word segmentation tasks and matching an object to a label in artificial languages specifically created for the tests. These studies concluded that children with SLI performed worse than autistic children in all measures. However, when the groups were reorganised according to language skills, a new group of ASD was created—ASD-LI. Then, both children with SLI and autistic children with LI presented results below controls in fast mapping. In relation to statistical learning, children with SLI had poor results, unlike autistic children, which performed similarly to TD peers.

Syntactic Skills

Studies that addressed syntax also show completely different methodologies. Fortunato-Tavares et al. ( 2015 ) verified that children with SLI, HFA and Down syndrome have an overall deficit in structuring the syntactic relations necessary for sentence comprehension, also showing a similar performance in the tasks where working memory demands are higher. Even so, children with SLI perform better in understanding ambiguous sentences compared with autistic children (Ishihara et al., 2015 ). Craig and Trauner ( 2017 ), in turn, characterised differences in the use of language in children with SLI and HFA, by analysing verbal responses on standardised tests, verifying that changes in syntactic expression are completely different in terms of SLI and ASD. In order to prove the existence of a link between syntactic and ToM skills, Durrleman et al. ( 2017 ) verified that children with SLI, autistic children and TD children with similar skills in production of sentences also showed comparable performances regarding ToM.

Pragmatic Skills

Speech planning and production difficulties in autistic children and children with SLI were explored by Gorman et al. ( 2016 ) through the analysis of the use of ‘uh’ and ‘um’ fillers during the application of the Autism Diagnostic Observation Schedule ( ADOS ) instrument. The group with ASD revealed a lower rate in the use of these fillers. Similar results were observed among autistic individuals and the control group.

Oral Language Comprehension and Production

Oral language comprehension and production skills, in general, are addressed by four studies. Developmental trajectories in children with language impairments were explored by Roy and Chiat ( 2014 ). The authors concluded that the majority of children who showed no differences in all assessed skills among them at the first evaluation moment (between 2 and 4 years old) presented differences at the third evaluation moment (between 9 and 11 years old): social communication deficits (11%), language impairment (27%), both difficulties (20%) and neither of them (42%). They also found that these difficulties emerged gradually.

Differences between language comprehension and production in autistic children and children with DLD, aged between 18 months and 4½ years old, were investigated by Seol et al. ( 2014 ). They verified that the group with DLD had superior comprehension skills, that the two groups did not reveal significant differences in terms of production and that language comprehension difficulties in the ASD group were more pronounced in younger children.

In the same line, but with children between 7 and 11 years old, Ramírez-Santana et al. ( 2019 ) examined and compared language phenotypes in individuals with SLI and autistic individuals, focusing on expressive and receptive language, as well as on core language and other specific skills, more specifically related to language content and language structure. The authors verified that there are no significant differences between the studied populations in all evaluated areas, concluding that there might be an overlap between language phenotypes.

To further elucidate the behavioural and linguistic profile associated with SPCD, HFA and SLI, Gibson et al. ( 2013 ) resorted to standardised measures to evaluate three groups of school-age children, each with one of the mentioned diagnoses. The authors verified that (1) there was a pattern of increasingly severe social difficulties observed in SLI, SPCD and HFA groups; (2) there are similarities in social communication and social interactional difficulties between HFA and PLI, but they are distinguished by the presence or absence of restricted and repetitive behaviours; (3) children with SLI are distinguished from other groups by the absence of social impairment; (4) increased expressive language ability was important for distinguishing SPCD from SLI, and decreased expressive language ability differentiated SPCD from HFA; (5) both the SLI and PLI groups showed advantage for receptive over expressive language; and (6) for children with HFA, receptive and expressive language were at similar levels, with a slight but nonsignificant tendency towards superior expressive language.

In Table 2 , we present the selected studies and their conclusion, regarding the existence of identical aspects in the populations under study. The first 5 studies evaluate PWM. All have similar methodologies: groups of pseudowords were formed, according to the number of syllables (between 3 and 5), in increasing order of length. The children immediately repeated the pseudowords perceived, and their response was recorded and later transcribed. The study by Tager-Flusberg ( 2015 ) also assesses sound discrimination of pseudowords: after the repetition test, pairs of pseudowords (previously recorded on the computer) were presented, and the children had to select ‘yes’ or ‘no’ on the monitor to indicate if they were the same. Of all the studies that evaluate this competence, Tager-Flusberg’s ( 2015 ) is the only one which concludes the existence of phenotypic overlap between ASD and SLI.

The remaining thirteen studies address different semantic, syntactic, pragmatic, general language comprehension and production skills by using completely different methodologies. Only six of these studies (Riches et al., 2012 ; McGregor et al., 2012 ; Heabig et al., 2015 ; Fortunato-Tavares et al., 2015 ; Durrleman et al., 2017 ; Ramírez-Santana et al., 2019 ) conclude that there are similarities between ASD and SLI.

Table 3 and Fig.  3 provide a clearer view of linguistic areas where language phenotypes overlap. Semantics is clearly the area in which most authors conclude that there are similarities between SLI and ASD-LI.

Comparison between the results of the 18 studies, based on the observation of similar or different linguistic performances

The majority of studies selected for this review focus on phonological working memory (pseudoword repetition), semantics (lexical processing, lexical knowledge, comprehension of compound nouns and lexical acquisition), syntax (sentence comprehension and syntactic expression) and pragmatics (social and communicative intent).

Working memory is highly associated with pragmatic competences and discourse comprehension in autistic individuals (Schuh et al., 2016 ). PWM, as well as receptive vocabulary, are considered significant predictors of fast mapping abilities in individuals with SLI (Alt & Plante, 2006 ; Jackson et al., 2016 ). PWM has thus been proposed as a reliable measure to assess language, being extremely useful for the identification of SLI/DLD (McDonald & Oetting, 2019 ). Thus, if there is a phenotypic overlap between this pathology and ASD, both groups have difficulties in performing these tasks. We found only one study (Tager-Flusberg, 2015 ) identifying a comorbidity between SLI and ASD-LI, and this result can be explained by the heterogeneity existing among these individuals with regard to their auditory discrimination skills, since these are highly correlated with pseudo-repetition skills and verbal production (Ebbels et al., 2012 ; Tager-Flusberg, 2015 ). So, as Ebbels et al. ( 2012 ) suggested, there is a deficit with phonology per se, rather than a deficit with phonological short-term memory or storage. Therefore, we conclude that children of the studied ages presenting similar problems in pseudoword auditory discrimination will also show similar difficulties in PWM.

The remaining results are in line with those obtained by Heaton et al. ( 2018 ), who investigated the impact of auditory short-term memory impairments in musical perception (assuming that music, like language, relies on auditory memory). Heaton et al. ( 2018 ), as well as Hill et al. ( 2015 ), Riches et al. ( 2011 ), Taylor et al. ( 2014 ) and Williams et al. ( 2013 ), verified that participants with SLI performed at significantly lower levels than the ASD-LI and TD groups. It is thus possible to conclude that there is a general deficit in terms of phonological skills which covers all individuals with SLI.

The work of Haebig et al. ( 2017 ), which assesses lexical acquisition processes, shows that children with SLI have more difficulties in statistical learning and fast mapping than autistic children and TD children. This result is in line with published literature. Jackson et al. ( 2016 ) found that word-learning difficulties of children with SLI may originate at fast mapping, in the scope of procedural deficit hypothesis (PDH). According to the PDH, those aspects of lexical acquisition and processing that rely on procedural sequential memory, namely the organisation and processing of lexical-phonological information, are impaired in children with SLI (Mainela-Arnold & Evans, 2014 ). However, we found that when the ASD group is divided according to language characteristics (ASD and ASD-LI), it is possible to observe that autistic individuals with LI also present difficulties in fast mapping. This result highlights the heterogeneity within the ASD group and reinforces the idea that it is extremely important to look at linguistic characteristics, rather than just at diagnoses. We can thus conclude that, in this ability, there is an overlap of linguistic phenotypes. The results of the study by Haebig et al. ( 2015 ) are also in line with these conclusions, attesting that lexical-semantic knowledge is similarly organised in all children who are matched on receptive vocabulary knowledge, regardless of the clinical group to which they belonged. Thus, once again, we can say that language skills go beyond diagnosis in their degree of importance. This work also suggests that lexical-semantic knowledge in autistic children and children with SLI may be immature but follows similar organisation of knowledge as in TD children, which corroborates published literature, referring to a delayed speed of processing in autistic children (e.g. Either DiStefano et al., 2019 ). However, in children/individuals with SLI, semantic processing is considered atypical (e.g. Pijnacker et al., 2017 ). Haebig et al. ( 2015 ) bring ASD-LI and SLI closer, suggesting an overlap between the diagnoses whenever children are matched by vocabulary level.

The studies by McGregor et al. ( 2012 ) and Riches et al. ( 2012 ) also allow us to notice an overlap in language phenotypes. That is, there are clear similarities between SLI and ASD in terms of vocabulary, word definition and association, recalling sentences from a given word (McGregor et al., 2012 ) and understanding compound nouns (Riches et al., 2012 ).

The objectives and methodologies of the studies addressing these abilities are very different. The results obtained by Ishihara et al. ( 2015 ) revealing better syntactic comprehension abilities in the group with SLI are considered to be expected, since this skill is also related to the semantic-pragmatic interface, which is a weak area in ASD (Westerveld & Roberts, 2017 ). Taking into account that the understanding of ambiguity was specifically assessed, the difficulties in semantic integration (Coderre et al., 2017 ) and the weaknesses of abstraction and conceptual generalisation of autistic children (Naigles et al., 2011 ), as well as their specific difficulties in oral narrative comprehension (Norbury et al., 2014 ) and in relative clause comprehension (Durrleman et al., 2015 ; Garrido et al., 2017 ), did not allow a good performance in the assessment. Through these results, we can verify the existence of distinct developmental profiles at the level of syntactic comprehension in individuals with SLI and autistic individuals.

Fortunato-Tavares et al. ( 2015 ), in turn, found an overall deficit in structuring syntactic relations necessary for sentence comprehension in SLI, HFA and Down syndrome. However, children with SLI exhibited similar performance to the children with Down syndrome and HFA when working memory demands were higher. In fact, there are several studies noticing the syntactic fragilities in SLI and ASD (e.g. Brynskov et al., 2017 ; Davies et al., 2016 ), being these results of special interest due to the importance of working memory. Also, Ebbels et al. ( 2012 ) concluded that SLI difficulties with tense marking were related to phonological short-term memory. Thus, we can notice that working memory, in general, assumes an important role, and that children with similar skills at this level also reveal an overlap in linguistic phenotypes.

Regarding morphosyntactic production, the results differ between studies. While Durrleman et al. ( 2017 ) conclude that there are similarities between individuals with SLI and autistic individuals, according to Craig and Trauner ( 2017 ), there are significant differences between the two groups, since individuals with SLI present more grammatical errors. Firstly, it is important to remember that the results obtained by Durrleman et al. ( 2017 ) show that, in the three groups, individuals with similar skills in the production of sentences also show comparable performances regarding ToM. Spanoudis ( 2016 ) verified that language skills and ToM are related and that syntactic and pragmatic abilities contributed significantly to the prediction of ToM performance in children with SLI. Thus, we found that children with SLI, autistic children and TD peers should have similar syntactic and pragmatic characteristics, and hence the similar results in ToM. It is also important to mention that this study was developed resorting to visual stimuli, which may have possibly helped decrease the errors in the group with SLI (Harper-Hill et al., 2014 ). In the work by Craig and Trauner ( 2017 ), only verbal stimuli are given in order to elicit children’s responses, which may be the reason for the high number of errors produced by individuals with SLI. We can conclude that the assessment of linguistic domains and the type of stimuli used are more important than the diagnosis.

The results of the studies described above emphasise the heterogeneity that exists in SLI and ASD, showing that a partial overlap of morphosyntactic phenotypes in both groups cannot be ruled out. This possibility is corroborated by neurological studies. On the one hand, some suggest that ASD-LI and SLI share neurodevelopmental changes in the cortical-cerebellar circuits that manage motor control and language, cognition, working memory and attention processing (Hodge et al., 2010 ). On the other hand, they conclude that there are different neuroanatomical substrates for language deficits in both disorders (Verhoeven et al., 2012 ), since different mechanisms for microstructural white matter alterations are observed in both groups (Roberts et al., 2014 ).

Similar results were observed among individuals with SLI and in the control group regarding the use of filler ‘um’, in contrast with the low rate of the fillers use presented by the ASD group (Gorman et al., 2016 ). These results are in line with the literature which, on the one hand, says that, during discourse, pauses can be filled with words like ‘um’ or ‘uh’, being that ‘um’ is used to signal longer pauses, and may correlate with greater social communicative sophistication than ‘uh’ (Parish-Morris et al., 2017 ). On the other hand, they agree with the results obtained by Irvine et al. ( 2016 ), which relate the low use of ‘um’ with the autism severity. This result allows us to suggest a specific pragmatic impairment in ASD.

With respect to oral language comprehension and production, in general, the conclusions are controversial. The results obtained by Seol et al. ( 2014 ) lead to the conclusion that there are different developmental language trajectories in autistic individuals and DLD individuals, since the first group reveals better production skills, compared to the second. These results corroborate those obtained by Davidson and Weismer ( 2017 ), who identified a greater impairment in language comprehension, compared to production, in the early years of life of autistic children. This characteristic affords the possibility of distinguishing autistic children from those presenting delayed language developments. Roy and Chiat’s ( 2014 ) work allows us to confirm that both groups have different developmental trajectories, considering that children who initially had the same language pathology evolve to completely different diagnoses. Conversely, Ramírez-Santana et al. ( 2019 ) verified that there are no significant differences between SLI and ASD in all evaluated areas, suggesting an overlap between language phenotypes. We consider that the discrepant results between these studies are due most probably to the age of the population evaluated. Whereas in the Seol et al. ( 2014 ) work, the groups were between 1 and 4 years old, in the study by Ramírez-Santana et al. ( 2019 ), the ages were between 7 and 11 years old. We can thus conclude that most likely, the large differences initially noted in the two populations are dissipating with age.

Finally, Gibson et al. ( 2013 ) verified that SPCD, HFA and SLI are distinguishable in terms of social difficulties, RRBIs and language development. These results are opposite to the obtained by Ramírez-Santana et al. ( 2019 ) and, with respect to language, unexpected. The ages of the groups are comparable in both studies and the instrument used to access language skills was the same ( Clinical evaluation of language fundamentals UK version 4 [CELF-4] ). Thus, the differences found are probably due to specific differences of the individuals, which leads us to conclude that this field needs further investigation.

Limitations and Future Directions for Research

Bearing in mind that this is a review that aims to bring together studies wherein there is a diagnostic assessment of the subjects, the constant change in the nomenclature of diagnoses and, specifically, the recent creation of the SPCD diagnosis may have been important limitations in this study. Future work should be performed through spontaneous speech, to better characterise language profiles and investigate if these similarities and differences persist over time. It is also important to analyse semantic, syntactic and pragmatic skills through samples of spontaneous speech, and understand not only how they work but also how they correlate with each other. It was clear in this work that phonology and working memory are related and that their development has repercussions in other areas, mainly in semantics and, therefore, in pragmatics. This aspect should also be studied. Language and cognition are complex systems, in which some skills depend on and are highly related to others, and this understanding is extremely important for the construction and implementation of rehabilitation programs by speech-language pathologists. The lack of spontaneous discourse analysis prevents us from realising how linguistic systems really work, and this should be the main objective for the future.

Conclusions

After analysing different research fields, we have noticed that some are clearly more explored than others, and that PWM is the most studied area, followed by semantics. This review draws our attention to the effects of heterogeneity within the diagnoses studied and to the specific characteristics of individuals, which may change the results obtained in different extents. It is thus possible to conclude that, when individuals in the studied groups are matched according to some language or cognitive skills (e.g. auditory discrimination, vocabulary, working memory, ToM), they will also show similar characteristics in other language domains. Therefore, it is extremely important to look at linguistic characteristics, rather than just at diagnoses, mainly when we are comparing two or more groups. Moreover, the type of stimuli used is extremely important, completely conditioning the result. The results of the analysed studies also suggest that the great differences between individuals with SLI and autistic individuals in their early years of life probably dissipate with age, resulting in similar language phenotypes in school age. Probably due to being a recent diagnosis, only one study was found that compares SPCD with SLI and ASD.

This literature review allowed a better comprehension of language characteristics and dynamics in autistic individuals and individuals with SLI and SPCD. This comprehension is central to the development of effective intervention programs by speech and language therapists. No studies were identified that analysed language skills through spontaneous speech, the best way to assess language development (Lopes-Herrera & Almeida, 2008 ). Most of the studies are based on standardised tests or subtests pulled out from different assessments. However, there are discourse impairments that may not be identified by measures that focus on individual words and sentences (Volden et al., 2017 ).

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Félix, J., Santos, M.E. & Benitez-Burraco, A. Specific Language Impairment, Autism Spectrum Disorders and Social (Pragmatic) Communication Disorders: Is There Overlap in Language Deficits? A Review. Rev J Autism Dev Disord 11 , 86–106 (2024). https://doi.org/10.1007/s40489-022-00327-5

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