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Split-Brain: What We Know Now and Why This is Important for Understanding Consciousness

Edward h. f. de haan.

1 Department of Psychology, University of Amsterdam, Amsterdam, the Netherlands

Paul M. Corballis

2 School of Psychology, University of Auckland, Auckland, New Zealand

Steven A. Hillyard

3 School of Health Sciences, University of California Dan Diego, La Jolla, CA USA

Carlo A. Marzi

4 School of Medicine and Surgery, University of Verona, Verona, Italy

5 Sackler Centre for Consciousness Science, Sussex University, Brighton, UK

Victor A. F. Lamme

6 Klinik für Neurologie, Universitätsklinikum Köln, Kerpener Str, 62 Köln, Germany

7 Dipartimento di Medicina Sperimentale e Clinica, Via Tronto 10/A, 60020 Ancona, Italy

Elizabeth Schechter

8 Department of Philosophy, Washington University, St. Louis, MO USA

9 Department of Philosophy, Monash University, Melbourne, Australia

Michael Corballis

Recently, the discussion regarding the consequences of cutting the corpus callosum (“split-brain”) has regained momentum (Corballis, Corballis, Berlucchi, & Marzi, Brain , 141 (6), e46, 2018 ; Pinto et al., Brain, 140 (5), 1231–1237, 2017a ; Pinto, Lamme, & de Haan, Brain, 140 (11), e68, 2017 ; Volz & Gazzaniga, Brain , 140 (7), 2051–2060, 2017 ; Volz, Hillyard, Miller, & Gazzaniga, Brain , 141 (3), e15, 2018 ). This collective review paper aims to summarize the empirical common ground, to delineate the different interpretations, and to identify the remaining questions. In short, callosotomy leads to a broad breakdown of functional integration ranging from perception to attention. However, the breakdown is not absolute as several processes, such as action control, seem to remain unified. Disagreement exists about the responsible mechanisms for this remaining unity. The main issue concerns the first-person perspective of a split-brain patient. Does a split-brain harbor a split consciousness or is consciousness unified? The current consensus is that the body of evidence is insufficient to answer this question, and different suggestions are made with respect to how future studies might address this paucity. In addition, it is suggested that the answers might not be a simple yes or no but that intermediate conceptualizations need to be considered.

Introduction

The term “split-brain” refers to patients in whom the corpus callosum has been cut for the alleviation of medically intractable epilepsy. Since the earliest reports by van Wagenen and Herren ( 1940 ) and Akelaitis ( 1941 , 1943 ) on the repercussions of a split-brain, two narratives have emerged. First and foremost is the functional description, pioneered by Gazzaniga, Sperry and colleagues (Gazzaniga, Bogen, & Sperry, 1963 ; Gazzaniga, Bogen, & Sperry, 1962 ; Sperry, 1968 ), in which the intricacies, the exceptions, the effects of different testing conditions, and the experimental confounds have been delineated by decades of extensive research with a relatively small group of patients (Berlucchi, Aglioti, Marzi, & Tassinari, 1995 ; Corballis, 1994 ; Corballis et al., 2010 ; Corballis, 2003 ; Luck, Hillyard, Mangun, & Gazzaniga, 1989 ; Pinto, Lamme, & de Haan, 2017b ; Volz, Hillyard, Miller, & Gazzaniga, 2018 ). It is important to note that even in this small group there are differences. In some patients all commissures were severed (“commissurotomy”), in others only the corpus callosum was cut (“callosotomy”) and some patients fall somewhere in between these two boundaries. Now, the search term “split-brain” results in a total of 2848 publications in the database of the Web-of-Science and 29,300 hits on Google Scholar, indicating a wealth of detailed information. The other depiction of split-brain patients entails the first-person perspective of the split-brain. In other words, “what is it like” to be a split-brain patient? It is especially this perspective that has captured the attention of the general press, popular science books and basic textbooks. By its nature, this second narrative lacks the detail of the functional description of the phenomenon, but it captures the intriguing question of how unity of consciousness is related to brain processes. Dominant in this description is the idea that in a split-brain each hemisphere houses an independent conscious agent. This notion, and particularly the concept of an isolated but conscious right hemisphere that is unable to express its feelings, desires or thinking due a lack of language, has captured the imagination (Gazzaniga, 2014 ).

It is important to clarify what we mean by unified consciousness. Here, we use the term in the sense of “subject unity” as defined by Bayne (Bayne & Chalmers, 2003 ; Bayne, 2008 ; Bayne, 2010 ). Subject unity is present if all the experiences generated in a system belong to one subject. In other words, if a system contains a first person perspective, then subject unity is preserved if that system only contains one such perspective, but subject unity is absent if the system contains multiple first person perspectives. Thus, in our definition of conscious unity, consciousness in a split-brain is split if each cortical hemisphere houses an independent conscious agent.

The view that consciousness is split in a split-brain has significantly impacted cognitive neuroscience at large. For instance, currently dominant theories about conscious awareness - the Integrated Information Theory (Tononi, 2005 ; Tononi, 2004 ) and the Global Neuronal Workspace Theory (Dehaene & Naccache, 2001 ; Dehaene, Kerszberg, & Changeux, 1998 ) - may be critically dependent on the validity of this view. Both theories imply that without massive communication between different subsystems, for instance cortical hemispheres, independent conscious agents arise. Thus, if the split consciousness view is invalid, these theories may be critically challenged.

The idea of split consciousness in a split-brain had its origin in the early split-brain studies (Gazzaniga, 1967 ; Gazzaniga, 1975 ; Gazzaniga et al., 1962 ; Sperry, 1968 ). These studies tested patients primarily in the two perceptual domains where processing is largely restricted to the contralateral hemisphere, that is vision and touch. In these early studies, stimuli, for instance objects, that were presented to the left hemisphere either physically in the right hand or as an image in the right visual half-field, could be readily named (as the left hemisphere is dominant for language) or pointed out with the right hand (which is controlled by the left hemisphere). The patient’s behavior became intriguing when the stimuli were presented in the left visual field or in the left hand. Now the patient, or at least the verbal left hemisphere, appeared oblivious to the fact that there had been a stimulus at all but was nevertheless able to select the correct object from an array of alternatives presented to the left hand or the left visual half-field (see Fig.  1 ). In a particularly dramatic recorded demonstration, the famous patient “Joe” was able to draw a cowboy hat with his left hand in response to the word “Texas” presented in his left visual half field. His commentary (produced by the verbal left hemisphere) showed a complete absence of insight into why his left hand had drawn this cowboy hat. Another astonishing example involved the same patient. MacKay and MacKay ( 1982 ) flashed a digit in the left visual field and trained the patient to play a version of ‘20 questions’ across hemispheres. The left hemisphere guessed the answer vocally, and the right hemisphere provided responses by pointing ‘up’ (meaning ‘guess a higher number’) or ‘down’ with the left hand. In this way the patient managed to vocalize the right answer. This suggests two independent conscious agents communicating with each other (one steering the left hand, the other agent controlling vocal expressions). However, note that an alternative interpretation is possible. Perhaps the patient knows the answer but finds it hard to vocalize. The ‘20 questions’ then simply help him in finding the correct vocalization.

One of the most well-known split-brain findings is that the patient claims verbally not to have seen the stimulus in the left visual field, yet indicates the identity of it with their left hand. This suggests that the left hemisphere (controlling verbal output) is blind to the left visual field, while the right hemisphere (controlling the left hand) does perceive it

Thus, these early observations suggested that there is no meaningful communication between the two hemispheres in split-brain patients. This led to the hypothesis that there might be two separate conscious agents, a left hemisphere that is able to talk to us and can explain what it sees and feels, and a mute right hemisphere that cannot communicate in language but that can nevertheless show that it has perceived and recognized objects and words. However, over time this view has eroded somewhat due to several anomalies (even right from the start) that may challenge this view.

Common Ground

An early observation, suggesting some remaining unification concerned what Joe Bogen called the “social ordinariness” of split-brain patients. Apart from a number of anecdotal incidents in the subacute phase following the surgery, these patients seem to behave in a socially ordinary manner and they report feeling unchanged after the operation (Bogen, Fisher, & Vogel, 1965 ; Pinto et al., 2017a ; R. W. Sperry, 1968 ; R. Sperry, 1984 ), although their extra-experiment behavior has not been systematically observed in great detail (Schechter, 2018 ). While the right hemisphere appears to be better at recognizing familiarity from faces, self-face recognition, that is the ability to realise immediately that a presented photograph represents you, appears to be available equally to both hemispheres in a split-brain patient (Uddin et al., 2008 ; Uddin, 2011 ). Thus, it seems unlikely that a mute but conscious right hemisphere would not have made itself known one way or the other. Thus, right from the start a paradox arose. The controlled lab tests suggested that consciousness is split in split-brain patients. Yet, everyday experiences of the patient and their close ones suggests that only one person exists in a split-brain. Additionally, it has been suggested that the two separate consciousnesses-hypothesis presumes that in the intact brain (before surgery) both hemispheres were conscious but connected via the corpus callosum, and they only became dissociable due to the operation. This casts doubt on the viability of the two consciousness view.

Crucially, the lab tests themselves were not always supportive of the split-consciousness view. Multiple experimental results showed that capacity for communication between the hemispheres varies both across patients and across tasks. For instance, a central observation in split-brain patients concerns the inability to compare visual stimuli across the midline. In other words, when one stimulus is presented to the left visual hemifield and the other to the right hemifield, the patient cannot accurately indicate whether both stimuli are the same or not, although they can do so when both stimuli are presented within one visual field. This is consistent with the notion that each hemisphere independently perceives the contralateral visual field, and that an intact corpus callosum is necessary for integration. Although there are indeed many examples of split-brain patients who are incapable of comparing stimuli across the midline, prominent examples can also be found of patients who can compare stimuli across the midline (Johnson, 1984 ; but see Seymour, Reuter-Lorenz, & Gazzaniga, 1994 ). This points to an important problem in the field, namely, individual differences. One aspect that may be important for individual differences is handedness differences. Variations in handedness may lead to differences in language capabilities in the right hemisphere, and could even underly differences in inter-hemispheric integration.

Moreover, under certain circumstances nearly all tested split-brain patients seem able to compare, or integrate, particular types of stimuli across the two visual half-fields (see Fig. ​ Fig.2). 2 ). An early demonstration of across hemifields integration is the study by Eviatar and Zaidel ( 1994 ). They showed that split-brain patients could accurately indicate the identity and shape of upper- and lower-case letters in either hemifield, regardless of with which hand they responded, for instance accurately identifying the letter A in the left visual field with the right hand. Yet these patients were mostly unable to compare these same stimuli across visual fields. In another experiment, two tilted lines were presented with a gap between them. The lines were positioned in such a way that extending them across the gap would either cause the lines to coincide or to run in parallel. When split-brain patients indicate whether the lines are parallel or coincident, they are highly accurate, even when both line segments are located in different half-fields (Corballis, 1995 ; Pinto, de Haan, Lamme, & Fabri, n.d. ; Sergent, 1987 ; Trevarthen & Sperry, 1973 ). Another example of visual integration across the midline involves apparent motion. When two dots are presented in succession at a short distance (2 to 14 visual degrees), split-brain patients are able to accurately indicate whether the dots create apparent motion, or that they were presented simultaneously or with delays too long to provoke apparent motion. Critically, they are able to do so even when one dot appears in the left visual field, and the other in the right visual field (Knapen et al., 2012 ; Naikar & Corballis, 1996 ; Ramachandran, Cronin-Golomb, & Myers, 1986 ). Clearly, under specific conditions, there is meaningful communication between the two hemispheres in the absence of the corpus callosum.

Although most split-brain patients cannot compare visual features such as shape and object identity across the midline, other features, such as good continuation of lines, and apparent motion, are integrated without a corpus callosum

Another observation that suggests some form of unity across the two visual half fields concerns detection and localization of stimulation, for instance, a brief flash (see, for example, an early study on the response times to light flashes with the ipsi- or contralateral hand: Clarke & Zaidel, 1989 ). Several investigations (Corballis, Corballis, Fabri, Paggi, & Manzoni, 2005 ; Pinto et al., 2017a ; Trevarthen & Sperry, 1973 ) have demonstrated convincingly that split-brain patients can accurately report the presence and location of stimuli for any position in the whole visual field, with either hand, and even verbally. Accurate detection and localization appears to be possible for all patients and all stimuli (different shapes, figures, equiluminant stimuli) tested so far. Thus, when patients in earlier studies said that they saw “nothing” when a stimulus was presented in the left visual half-field, they may have meant that they could see it but that they could not identify or retrieve the name of the object.

Other findings point to a crucial difference between the degree of lateralization of visual-perceptual processing and producing overt responses. Perception appears to be more split, while responding remains largely unified. Whether a stimulus appears in the left or the right visual hemifield strongly impacts performance of split-brain patients. However, response type (left hand, right hand or verbally) seems to have a much smaller, or no effect at all. For instance, Pinto et al., 2017a ) found that the split-brain patient was much better at matching pictures to sample stimuli in the left visual field. Yet, for the exact same stimuli matching pictures to verbal labels was vastly superior when the stimuli appeared in the right visual field. Crucially, response type did not play any role. The patient was better in matching pictures to sample for stimuli in the left visual field, even if they responded verbally or with the right hand. Similarly, Levy, Trevarthen, and Sperry ( 1972 ) presented split-brain patients with chimeric or composite faces, that is, one half-face in each visual field. Subsequently the patient either matched the chimeric face to sample, or attached a verbal label to it. Verbal matching was mostly based on the half-face in the right visual field, while matching to sample was mainly driven by the half-face in the left visual field. But crucially, the latter was the case, independent of whether the patient responded with the left or the right hand.

Thus, it seems that in split-brain patients perceptual processing is largely split, yet response selection and action control appear to be unified under certain conditions. This, by itself, does not prove whether a split-brain houses one or two conscious agents. One explanation could be that the split-brain houses two agents, each having their own experiences, who synchronize their behavioral output through various means. Another possible explanation is that a split-brain houses one agent who experiences an unintegrated stream of information who controls the entire body, comparable to watching a movie where sight and sound are out-of-sync. At any rate, these findings challenge the previously mentioned classic split-brain description, which is still found in reviews and text books (Gray, 2002 ; Wolman, 2012 ). In this classic characterization the patient indicates that they saw nothing when a stimulus appeared in the left visual field. Yet, to their own verbal surprise, the left hand correctly draws the stimulus. The aforementioned examples of unity in action control suggests that these effects may depend on the type and complexity of the response that is required.

Interpretations

There are three, not-mutually exclusive, hypotheses concerning the mechanisms involved in, seemingly, preserved unity in the split-brain. The first notion is that information is transferred subcortically. The second idea is that ipsilateral motor control underlies unity in action control. The third idea claims that information transfer is based on varies forms of inter-hemispheric collaboration, including subtle behavioral cues. The first proposal (Corballis Corballis, Berlucchi, & Marzi, 2018 ; de Haan et al., 2019 ; Pinto, Lamme, & de Haan, 2017b ; Pinto et al., 2017a ; Savazzi et al., 2007 ; Mancuso, Uddin, Nani, Costa, & Cauda, 2019 ) suggests that the multitude of subcortical connections that are spared during surgery are responsible for the transfer of information. As was initially pointed out by Trevarthen ( 1968 ) and Trevarthen and Sperry ( 1973 ) and recently stressed by Pinto, de Haan, and Lamme ( 2017a ) and Corballis et al. ( 2018 ), there are many commissures (white matter tracts that connect homologous structures on both sides of the central nervous system) and decussations (bundles that connect different structures on both sides) that link nuclei that are known to be involved in perceptual processing. The importance of these commisural connections for transferring visual information in split-brain patients has been highlighted by Trevarthen and Sperry ( 1973 ). Moreover, the role of these connections in a split-brain has recently been demonstrated by bilateral fMRI activations in the first somatosensory cortex, after unilateral stimulation of trunk midline touch receptors (Fabri et al., 2006 ) and in the second somatic sensory area after unilateral stimulation of hand pain receptors (Fabri, Polonara, Quattrini, & Salvolini, 2002 ). Uddin and colleagues used low-frequency BOLD fMRI resting state imaging to investigate functional connectivity between the two hemispheres in a patient in whom all major cerebral commissures had been cut (Uddin et al., 2008 ). Compared to control subjects, the patient’s interhemispheric correlation scores fell within the normal range for at least two symmetrical regions. In addition, Nomi and colleagues suggested that split-brain patients might rely particularly on dorsal and ventral pontine decussations of the cortico-cerebellar interhemispheric pathways as evidenced by increased fractional anisotropy (FA) on diffusion weighted imaging (Nomi, Marshall, Zaidel, Biswal, Castellanos, Dick, Uddin & Mooshagian, 2019). Interhemispheric exchange of information also seems to occur in the domain of taste sensitivity, activation of primary gustatory cortex in the fronto-parietal operculum was reported in both hemispheres after unilateral gustatory stimulation of the tongue receptors (Mascioli, Berlucchi, Pierpaoli, Salvolini, Barbaresi, Fabri, & Polonara, 2015 ). Note that patients may differ with respect to how many of these connections have been cut, and this might also explain some of the individual variance among patients. Moreover, in all patients subcortical structures remain intact. For instance, the superior colliculus is known to integrate visual information from both hemispheres and project information to both hemispheres (Meredith & Stein, 1986 ; Comoli et al., 2003 ). Such structures may support attentional networks, and may enable the right hemisphere to attend to the entire visual field. In turn, attentional unity could help in unifying cognitive and motor control, which may subserve ipsilateral motor control.

The second point concerns the ipsilateral innervation of the arms. Manual action is not strictly lateralized, and the proximal (but not the distal) parts of the arm are controlled bilaterally, although the ipsilateral contribution remains undetermined. This could explain why split-brain patients may respond equally well with both hands in certain experimental conditions (Corballis, 1995 ; Gazzaniga, Bogen, & Sperry, 1967 ; Pinto, de Haan, & Lamme, 2017a ). First, there is substantial evidence that bilateral cortical activations can be observed during unilateral limb movements in healthy subjects. In addition, ipsilesional motor problems in arm control have been observed in patients with unilateral cortical injuries, and finally there is evidence from electrocorticography with implanted electrodes for localization of epileptic foci showing similar spatial and spectral encoding of contralateral and ipsilateral limb kinematics (Bundy, Szrama, Pahwa, & Leuthardt, 2018 ). While these observations argue convincingly for a role in action control by the ipsilateral hemisphere, they do not prove that a hemisphere on it’s own can purposefully control the movements of the ipsilateral hand. Thus, the role of ipsilateral arm-hand control in explaining split-brain findings is currently not settled.

The third hypothesis argues that in addition to whatever direct neural communication may exist between the hemispheres, they may inform one another via strategic cross-cueing processes (Volz & Gazzaniga, 2017 ; Volz et al., 2018 ). The split-brain patients underwent surgery many years prior to testing, and the separated perceptual systems have had ample time to learn how to compensate for the lack of commissural connections. For example, subtle cues may be given by minimal movements of the eyes or facial muscles, which might not even be visible to an external observer but are capable of encoding, for example, the location of a stimulus for the hemisphere that did not “see” it. A cross-cueing mechanism might also allow one hemisphere to convey to the other which one of a limited set of known items had been shown (Gazzaniga & Hillyard, 1971 ; Gazzaniga, 2013 ).

Finally, it is possible to entertain combinations of the different explanations. For instance, it is conceivable that in the subacute phase following split-brain surgery the hemispheres are ineffective in communicating with each other. During this initial phase, phenomena such as an “alien hand” - that is a hand moving outside conscious control of the (verbal) person - may be present. In the ensuing period, the patients may have learned to utilize the information that is exchanged via subcortical connections, ipsilateral motor control or cross-cueing to coordinate the processing of the two hemispheres. In such a way, the patient may counteract some of the effects of losing the corpus callosum.

What do We Need to Know?

This paper aims to contribute to the agenda for the next decade of split-brain research. Full split-brain surgery is rare these days, and it is important that we try to answer the central questions while these patients are still available for study. In order to examine the variations between patients it would be useful to test as many of the available patients as possible with the same tests.

One important goal is to map out precisely how much functionality and information is still integrated across hemispheres in the split-brain, and what the underlying principles are. For instance, in some cases the two hemispheres seem to carry out sensory-motor tasks, such as visual search, independently from one another (Arguin et al., 2000 ; Franz, Eliassen, Ivry, & Gazzaniga, 1996 ; Hazeltine, Weinstein, & Ivry, 2008 ; Luck, Hillyard, Mangun, & Gazzaniga, 1994 ; Luck et al., 1989 ), while in other cases functions such as attentional blink, or attentional cueing, seem to be integrated across hemispheres (Giesbrecht & Kingstone, 2004 ; Holtzman, Volpe, & Gazzaniga, 1984 ; Holtzman, Sidtis, Volpe, Wilson, & Gazzaniga, 1981 ; Pashler et al., 1994 ; Ptito, Brisson, Dell’Acqua, Lassonde, & Jolicœur, 2009 ). An important challenge is to unveil why some cognitive functions can be carried out independently in the separated hemispheres while other functions engage both hemispheres. Furthermore, it is now clear that accurate detection and localization is possible across the whole visual field, and there is some evidence that even more information concerning visual images can be transferred between hemispheres. Although we have some understanding of what types of information can be transferred in the visual domain, our knowledge base in the somatosensory domain is much more limited. This is probably due to a bias throughout cognitive neuroscience and psychology, leading to a strong focus on vision in split-brain research. It is important to collect converging evidence by investigating the somatosensory system which is also strongly lateralized. Note that in somatosensory processing transfer between hemispheres (about 80% correct for the bimanual conditions) has been observed for basic same-different matching of real objects (Fabri, Del Pesce et al., 2005 ).

Another important goal is to obtain a more detailed description of the perceptual, cognitive and linguistic capabilities of the disconnected right hemisphere. For understanding unity of mind, two capabilities specifically are crucial. First, experiments investigating aspects of the conscious mind often go beyond simple visual processing, and future studies will thus critically depend on testing high-level cognitive abilities of both hemispheres. Specifically, language abilities, crucial for understanding questions and instructions, will likely play a pivotal role. Thus, the first question is to what extent the right hemisphere is capable of language processing. Note that complicated instructions (Gazzaniga, Smylie, Baynes, Hirst, & McCleary, 1984 ; Pinto et al., 2017a ; Zaidel, 1983 ), for instance relating to mental imagery (Johnson, Corballis, & Gazzaniga, 2001 ; Kosslyn, Holtzman, Farah, & Gazzaniga, 1985 ; Sergent & Corballis, 1990 ), seem to be well within the reach of the right hemisphere. Moreover, right hemisphere language capabilities seem to improve over time (Gazzaniga, Volpe, Smylie, Wilson, & LeDoux, 1979 ; Gazzaniga et al., 1996 ). Longitudinal language tests (for instance with a Token test: De Renzi & Vignolo, 1962 ) would further illuminate the extent of right hemisphere language processing.

Second, unveiling to what extent each hemisphere is capable of subserving consciousness at all seems relevant for unity of mind as well. If the disconnected right hemisphere can produce full-blown consciousness, then questions regarding unity of mind are clearly more pertinent then if the right hemisphere only produces minimal amounts of consciousness. Right hemisphere consciousness can be studied through novel neural paradigms (Bekinschtein et al., 2009 ; Casali et al., 2013 ; Pitts, Metzler, & Hillyard, 2014 ; Shafto & Pitts, 2015 ). For instance, Bekinschtein et al. employed EEG to measure if the brain detected irregularities (as indicated by an event-related potential [ERP] signal called the P3) in different states of consciousness. They found that when consciousness was reduced, local irregularities were still detected - for instance after three high auditory tones a low tone evoked a P3. However, global irregularities - several times a low tone followed three high tones, then on the critical trial three high tones were followed by another high tone - did not evoke a P3 when consciousness was reduced. Crucially, when consciousness was unimpaired both local and global irregularities evoked a P3 response. Right hemisphere consciousness may also be studied in other patient groups where interhemispheric communication is hampered. One particularly interesting group are post-hemispherotomy patients (Lew, 2014 ). These patients have been surgically treated to disconnect an entire hemisphere (usually for intractable epilepsy), but unlike hemispherectomy patients the disconnected hemisphere remains in place in the cranium and remains vascularized.

Clearly, the central question, whether each hemisphere supports an independent conscious agent, is not settled yet. Novel paradigms in this respect could lead to progress. For instance, a pivotal question is whether each hemisphere makes its own decisions independent of the other hemisphere. If each hemisphere produces its own autonomous conscious agent then this should be the case. That is, if two agents are asked to freely choose a random number, then the odds that they consistently pick the same number are small. And vice versa, if each hemisphere makes its own conscious decisions, independent of the other hemisphere, then this seems to rule out unity of mind. Note that each hemisphere making its own decisions is different from information processing occurring independently per hemisphere. Unconscious information processing is almost certainly split across hemispheres in a split-brain. However, this does not prove that consciousness is split or unified. Even in a healthy brain, where consciousness is unified, many unconscious processes run independently, and in parallel.

One way to tackle the central question is by having the hemispheres respond to questions in parallel. Overt behavior most likely does not allow for this, due to bilateral motor control processes sketched earlier. However, perhaps parallel responding is possible if the hemispheres produce covert responses. For instance, the patient could be asked to pick one of four options and indicate their choice by carrying out certain content-specific mental imagery tasks. This imagery can then be decoded in parallel from each hemisphere using neuroimaging techniques (see Owen et al., 2006 for a similar approach with vegetative state patients). If each hemisphere harbors an autonomous conscious agent, then it is highly unlikely that the two hemispheres will consistently make the same choices. Thus, if the choices are uncorrelated across hemispheres, then this may critically challenge the unified mind view.

Another way to tackle the question of unified consciousness in the split-brain is to employ ERPs as markers of concurrent conscious processing in the left and right hemispheres. For instance, in one study (Kutas, Hillyard, Volpe, & Gazzaniga, 1990 ) visual targets were presented either separately to the left or right visual field or to both visual fields simultaneously. It was found that the P300 - a signal possibly reflecting conscious processing of a visual target (Dehaene & Changeux, 2011 ; Dehaene, Charles, King, & Marti, 2014 ; Salti, Bar-Haim, & Lamy, 2012 ) - was reduced for bilateral targets. This suggests some type of integration of conscious processing. Studies employing ERPs may indicate whether conscious processing is unified, while unconscious processing is split, which would be suggestive of unified consciousness.

Conclusions

In summary, the pivotal issue in split-brain research is whether dividing the brain divides consciousness. That is, do we find evidence for the existence of one, or two conscious agents in a split-brain? Note that intermediate results may be found. Perhaps some measures indicate unified consciousness while others do not. This would then provoke further interesting questions on the unity of consciousness. What are the crucial measures for unity of consciousness? If intermediate results are found, more unconventional possibilities should be entertained as well. For instance, although difficult to fathom, some philosophers have suggested that a split-brain does not contain one or two observers, but a non-whole number of conscious agents (Nagel, 1971 ; Perry, 2009 ), for instance one and a half first-person perspectives. If evidence for this position is found, then its implications would stretch beyond split-brain patients. It would suggest that our intuitions on the indivisibility of the experiential self may be mistaken. One way to think of this is as with the difference between conscious and unconscious processing. Perhaps this is not a dichotomous distinction, but a continuum between more or less conscious. Similarly, perhaps the existence of a first-person perspective is not dichotomous, but gradual as well. Another possibility is that a split-brain does contain a whole number of conscious agents, but that consciousness across these agents is only partially unified. That is, the agents share some conscious experiences and decisions, but not all (Lockwood, 1989 ; Schechter, 2014 ; Schechter, 2018 ; Schechter, 2013 ). Finally, another way to look at this is in terms of ‘dissociation’, as in depersonalization (Phillips et al., 2001 ; Sierra et al., 2002 ). Perhaps the number of agents is not altered, but the agent feels depersonalized in some situations, and therefore no longer feels that they control the actions, or even experience the information, that has just occurred in their brain.

New findings on the unity of consciousness in a split-brain could fundamentally impact currently dominant consciousness theories. Global Neuronal Workspace Theory (Dehaene & Naccache, 2001 ; Dehaene et al., 1998 ) asserts that consciousness arises when information that is processed in unconscious (or preconscious) modules is broadcast to a central ‘workspace’, primarily residing in frontal regions of the brain. Although not very explicit on the unity of consciousness in split-brain patients, Global Neuronal Workspace Theory seems to endorse the split consciousness idea, given that each hemisphere has its own prefrontal hub, enabling broadcasting of whatever information is processed in that hemisphere.

Integrated Information Theory (Tononi, 2005 ; Tononi, 2004 ) has specifically addressed the issue of split brain (for instance in Tononi, 2004 ). Integrated Information Theory asserts that ‘phi’, a measure of how integrated information is, determines the level of consciousness. The higher phi, the more conscious a system is. Moreover, local maxima in phi correspond to conscious agents. If in a system all subsystems are highly interconnected, then phi is highest for the system as a whole, and local maxima are absent. Thus, such a system produces only one conscious agent. However, if subsystems only exchange minimal amounts of information, then phi per subsystem is higher than phi for the system as whole. In such a case each subsystem creates its own conscious agent. In a split-brain, connectedness, that is integration of information, is much higher within than across hemispheres. Therefore, according to Integrated Information Theory consciousness should be split in a split-brain.

Recurrent Processing theory (Lamme & Roelfsema, 2000 ; Lamme, 2004 ; Lamme, 2010 ) argues for the independence of consciousness from attention, access, or report. This theory has addressed the issue of report specifically, making the case that consciousness and reportability, whether verbal or manual, should be viewed as entirely independent (Lamme, 2006 ; Tsuchiya, Wilke, Frässle, & Lamme, 2015 ). Crucially, this theory states that even in the normal mind, ‘islands’ of unattended yet conscious information reside (Lamme, 2006 ). In these cases, all the information, although functionally unintegrated, is nonetheless experienced by the same mind. Support for this view comes from findings in multiple object tracking (Pinto, Howe, Cohen, & Horowitz, 2010 ; Pinto, Scholte, & Lamme, 2012 ; Pylyshyn & Storm, 1988 ). Here, evidence indicates that when moving objects in two visual hemifields are tracked, attention is split (Howe, Cohen, Pinto, & Horowitz, 2010 ) and each hemisphere processes the relevant information in the contralateral visual field independently of the other (Alvarez & Cavanagh, 2005 ; Cavanagh & Alvarez, 2005 ; Drew, Mance, Horowitz, Wolfe, & Vogel, 2014 ). That is, the left hemisphere only tracks the right visual field and vice versa. Yet, although the visual information is not integrated across hemispheres, from a first person perspective, it seems clear that the subject experiences all moving objects across the entire visual field. Another example of the dissociation between consciousness and reportability is the so-called partial report paradigm (Pinto et al., 2017b ; Pinto, Sligte, Shapiro, & Lamme, 2013 ; Sligte, Scholte, & Lamme, 2008 ; Sperling, 1960 ). In these paradigms subjects seem to remember more than they can report. Thus, reportability and consciousness are dissociated. Perhaps in split-brain patients this dissociation is simply more pronounced. That is, consciousness remains unified, but reportability has become more dissociated, thereby inducing the appearance of two independent agents. In sum, according to the Recurrent Processing theory, integration of information is not needed for a unified mind, implying that the mind may remain unified when the brain is split. Thus, different theories of consciousness have different predictions on the unity of mind in split-brain patients, and await the results of further investigation into this intriguing phenomenon.

Acknowledgements

This work was supported by an Advanced Investigator Grant by the European Research Council (ERC grant FAB4V (#339374) to EdH and a Templeton grant (ID# 61382, "Towards understanding a unified mind") to YP and VL.

Compliance with Ethical Standards

The authors report no competing interests.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Roger Sperry’s Split Brain Experiments (1959–1968)

Editor's note: Sarah Walls created the above image for this article. You can find the full image and all relevant information here .

In the 1950s and 1960s, Roger Sperry performed experiments on cats, monkeys, and humans to study functional differences between the two hemispheres of the brain in the United States. To do so he studied the corpus callosum, which is a large bundle of neurons that connects the two hemispheres of the brain. Sperry severed the corpus callosum in cats and monkeys to study the function of each side of the brain. He found that if hemispheres were not connected, they functioned independently of one another, which he called a split-brain. The split-brain enabled animals to memorize double the information. Later, Sperry tested the same idea in humans with their corpus callosum severed as treatment for epilepsy, a seizure disorder. He found that the hemispheres in human brains had different functions. The left hemisphere interpreted language but not the right. Sperry shared the Nobel Prize in Physiology or Medicine in 1981for his split-brain research.

Sperry also studied other aspects of brain function and connections in mammals and humans, beyond split-brains, in 1940s and 1950s. In 1963, he developed the chemoaffinity hypothesis, which held that the axons, the long fiber-like process of brain cells, connected to their target organs with special chemical markers. This explained how complex nervous systems could develop from a set of individual nerves. Sperry then also studied brain patterns in frogs, cats, monkeys, and human volunteers. Sperry performed much of his research on the split-brain at California Institute of Technology, or Caltech, in Pasadena, California, where he moved in 1954.

Sperry began his research on split-brain in late 1950s to determine the function of the corpus callosum. He noted that humans with a severed corpus callosum did not show any significant difference in function from humans with intact corpus callosum, even though their hemispheres could not communicate due to the severing of the corpus callosum. Sperry postulated that there should be major consequences from cutting the brain structure, as the corpus callosum connected the two hemispheres of the brain, was large, and must have an important function. Sperry began designing experiments to document the effects of a severed corpus callosum. At the time, he knew that each hemisphere of the brain is responsible for movement and vision on the opposite side of the body, so the right hemisphere was responsible for the left eye and vice versa. Therefore, Sperry designed experiments in which he could carefully monitor what each eye saw and therefore what information is was going to each hemisphere.

Sperry experimented with cats, monkeys, and humans. His experiments started with split-brain cats. He closed one of their eyes and presented them with two different blocks, one of which had food under it. After that, he switched the eye patch to the other eye of the cat and put the food under the other block. The cat memorized those events separately and could not distinguish between the blocks with both eyes open. Next, Sperry performed a similar experiment in monkeys, but made them use both eyes at the same time, which was possible due to special projectors and light filters. The split-brain monkeys memorized two mutually exclusive scenarios in the same time a normal monkey memorized one. Sperry concluded that with a severed corpus callosum, the hemispheres cannot communicate and each one acts as the only brain.

Sperry moved on to human volunteers who had a severed corpus callosum. He showed a word to one of the eyes and found that split-brain people could only remember the word they saw with their right eye. Next, Sperry showed the participants two different objects, one to their left eye only and one to their right eye only and then asked them to draw what they saw. All participants drew what they saw with their left eye and described what they saw with their right eye. Sperry concluded that the left hemisphere of the brain could recognize and analyze speech, while the right hemisphere could not.

In the 1960s when Sperry conducted his split-brain research on humans, multiple scientists were studying brain lateralization, the idea that one hemisphere of the brain is better at performing some functions than the other hemisphere. However, researchers did not know which tasks each side of the brain was responsible for, or if each hemisphere acted independently from the other.

Sperry describes his research in cats in the article "Cerebral Organization and Behavior" published in 1961. To test how the cutting of the corpus callosum affected mammals, Sperry cut the corpus callosum of multiple cats and had them perform some tasks that involved their vision and response to a visual stimulus. After severing each cat´s corpus callosum, he covered one of the cat´s eyes to monitor with which eye the cat could see. Sperry could switch the eye patch from one eye to the other, depending on which visual field he wanted the cat to use. Next, Sperry showed the cats two wooden blocks with different designs, a cross and a circle. Sperry put food for the cat under one of the blocks. He taught the cats that when they saw the blocks with one eye, for instance, the right eye, the food was under the circle block, but when they saw it with the left eye, the food was under the block with a cross. Sperry taught the cats to differentiate between those two objects with their paws, pushing the correct wooden block away to get the food.

When Sperry removed the eye patch and the cats could see with both eyes, he performed the same experiment. When the cats could use both eyes, they hesitated and then chose both blocks almost equally. The right eye connects to the left hemisphere and the left eye connects to the right hemispheres. Sperry suspected that since he cut the corpus callosum in those cats, the hemispheres could not communicate. If the hemispheres could not communicate and the information from one eye only went to one hemisphere, then only that hemisphere would remember which block usually had food under it. From that, Sperry concluded that the cats remembered two different scenarios with two different hemispheres. He suspected that the cats technically had two different brains, as their hemispheres could not interact and acted as if the other one did not exist.

Sperry performed a similar experiment with monkeys, in which he also cut their corpus callosum. He wanted to test if both hemispheres could operate at the same time, even though they were not connected. That required separation of visual fields, or making sure that the right eye saw a circle, while the left eye saw a cross, like in the cat experiment, but without an eye patch and both eyes would see something at the same time instead of interchanging between the open eyes. Sperry solved that by using two projectors that were positioned side-by-side at an angle and showed mutually exclusive images. For example, the projector on the right showed a circle on the left and a cross on the right, while the projector on the left showed a cross on the left and a circle on the right. Sperry placed special light filters in front of each of the monkey´s eyes. The light filters made it so that each eye saw the images from only one of the projectors. That meant one of the eyes saw the circle on the right and the cross on the left, while the other eye saw the cross on the right and the circle on the left. From his experiments with cats, Sperry knew that there was no sharing of information from right and the left hemispheres, so he made the monkeys memorize two different scenarios at the same time.

The left eye saw a scenario where food would be dispersed when the monkey pressed the button corresponding to a cross, while the right eye saw a scenario where food would be dispersed when the monkey pressed a button corresponding to a circle. Ultimately, it was the same button, but the eyes saw it differently because of two projectors and special light filters. Sperry concluded that both hemispheres of the brain were learning two different, reversed, problems at the same time. He noted that the split-brain monkeys learned two problems in the time that it would take a normal monkey to learn one, which supported the assumption that the hemispheres were not communicating and each one was acting as the only brain. That seemed as a benefit of cutting corpus callosum, and Sperry questioned whether there were drawbacks to the procedure.

Sperry performed the next set of experiments on human volunteers, who had their corpus callosum severed previously due to outside factors, such as epilepsy. Sperry asked volunteers to perform multiple tests. From his previous experiments with cats and monkeys, Sperry knew that one, the opposite, hemisphere of the brain would only analyze information from one eye and the hemispheres would not be able to communicate to each other what they saw. He asked the participants to look at a white screen with a black dot in the middle. The black dot was the dividing point for the fields of view for a person, so the right hemisphere of the brain analyzed everything to the left of the dot and the left hemisphere of the brain analyzed everything that appeared to the right of the dot. Next, Sperry showed the participants a word on one side of the black dot for less than a second and asked them to tell him what they saw. When the participants saw the word with their right eye, the left hemisphere of the brain analyzed it and they were able to say what they saw. However, if the participants saw the word with their left eye, processed by right hemisphere, they could not remember what the word was. Sperry concluded that the left hemisphere could recognize and articulate language, while the right one could not.

Sperry then tested the function of the right hemisphere. He asked the participants of the same experiment that could not remember the word because it was in the left visual field to close their eyes and draw the object with their left hand, operated by the right hemisphere, to which he presented the word. Most people could draw the picture of the word they saw and recognize it. Sperry also noted that if he showed the word to the same visual field twice, then the person would recognize it as a word they saw, but if he showed it to the different visual fields, then the participants would not know that they saw the word before. Sperry concluded that the left hemisphere was responsible not only for articulating language, but also for understanding and remembering it, while the right hemisphere could only recognize words, but was not able to articulate them. That supported the previously known idea that the language center was in the left hemisphere.

Sperry performed another similar experiment in humans to further study the ability of the right hemisphere to recognize words. During that experiment, Sperry asked volunteers to place their left hand into a box with different tools that they could not see. After that, the participants saw a word that described one of the objects in the box in their left field of view only. Sperry noted that most participants then picked up the needed object from the box without seeing it, but if Sperry asked them for the name of the object, they could not say it and they did not know why they were holding that object. That led Sperry to conclude that the right hemisphere had some language recognition ability, but no speech articulation, which meant that the right hemisphere could recognize or read a word, but it could not pronounce that word, so the person would not be able to say it or know what it was.

In his last series of experiments in humans, Sperry showed one object to the right eye of the participants and another object to their left eye. Sperry asked the volunteers to draw what they saw with their left hand only, with closed eyes. All the participants drew the object that they saw with their left eye, controlled by the right hemisphere, and described the object that they saw with their right eye, controlled by the left hemisphere. That supported Sperry´s hypothesis that the hemispheres of brain functioned separately as two different brains and did not acknowledge the existence of the other hemisphere, as the description of the object did not match the drawing. Sperry concluded that even though there were no apparent signs of disability in people with a severed corpus callosum, the hemispheres did not communicate, so it compromised the full function of the brain.

Sperry received the 1981 Nobel Prize in Physiology or Medicine for his split-brain research. Sperry discovered that the left hemisphere of the brain was responsible for language understanding and articulation, while the right hemisphere could recognize a word, but could not articulate it. Many researchers repeated Sperry´sf experiments to study the split-brain patterns and lateralization of function.

• Sperry, Roger W. "Cerebral Organization and Behavior." Science 133 (1961): 1749–57. http://people.uncw.edu/puente/sperry/sperrypapers/60s/85-1961.pdf (Access December 8, 2017).
• Sperry, Roger W. "Hemisphere Deconnection and Unity in Conscious Awareness." American Psychologist 28 (1968): 723–33. http://people.uncw.edu/Puente/sperry/sperrypapers/60s/135-1968.pdf (Access December 8, 2017).
• Sperry, Roger W. "Split-brain Approach to Learning Problems." In The Neurosciences: A Study Program , eds. Gardner C. Quarton, Theodore Melnechuk, and Francis O. Schmitt, 714–22. New York: Rockefeller University Press, 1967. ttp://people.uncw.edu/puente/sperry/sperrypapers/60s/130-1967.pdf (Accessed November15, 2017).
• "The Split Brain Experiments." Nobelprize.org . https://www.nobelprize.org/educational/medicine/split-brain/background.html (Accessed May 3, 2017).

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Split-Brain: What We Know Now and Why This is Important for Understanding Consciousness

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• Published: 12 May 2020
• Volume 30 , pages 224–233, ( 2020 )

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• Edward H. F. de Haan 1 ,
• Paul M. Corballis 2 ,
• Steven A. Hillyard 3 ,
• Carlo A. Marzi 4 ,
• Anil Seth 5 ,
• Victor A. F. Lamme 1 ,
• Lukas Volz 6 ,
• Mara Fabri 7 ,
• Elizabeth Schechter 8 ,
• Tim Bayne 9 ,
• Michael Corballis 2 &
• Yair Pinto 1  

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Recently, the discussion regarding the consequences of cutting the corpus callosum (“split-brain”) has regained momentum (Corballis, Corballis, Berlucchi, & Marzi, Brain , 141 (6), e46, 2018 ; Pinto et al., Brain, 140 (5), 1231–1237, 2017a ; Pinto, Lamme, & de Haan, Brain, 140 (11), e68, 2017 ; Volz & Gazzaniga, Brain , 140 (7), 2051–2060, 2017 ; Volz, Hillyard, Miller, & Gazzaniga, Brain , 141 (3), e15, 2018 ). This collective review paper aims to summarize the empirical common ground, to delineate the different interpretations, and to identify the remaining questions. In short, callosotomy leads to a broad breakdown of functional integration ranging from perception to attention. However, the breakdown is not absolute as several processes, such as action control, seem to remain unified. Disagreement exists about the responsible mechanisms for this remaining unity. The main issue concerns the first-person perspective of a split-brain patient. Does a split-brain harbor a split consciousness or is consciousness unified? The current consensus is that the body of evidence is insufficient to answer this question, and different suggestions are made with respect to how future studies might address this paucity. In addition, it is suggested that the answers might not be a simple yes or no but that intermediate conceptualizations need to be considered.

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Introduction

The term “split-brain” refers to patients in whom the corpus callosum has been cut for the alleviation of medically intractable epilepsy. Since the earliest reports by van Wagenen and Herren ( 1940 ) and Akelaitis ( 1941 , 1943 ) on the repercussions of a split-brain, two narratives have emerged. First and foremost is the functional description, pioneered by Gazzaniga, Sperry and colleagues (Gazzaniga, Bogen, & Sperry, 1963 ; Gazzaniga, Bogen, & Sperry, 1962 ; Sperry, 1968 ), in which the intricacies, the exceptions, the effects of different testing conditions, and the experimental confounds have been delineated by decades of extensive research with a relatively small group of patients (Berlucchi, Aglioti, Marzi, & Tassinari, 1995 ; Corballis, 1994 ; Corballis et al., 2010 ; Corballis, 2003 ; Luck, Hillyard, Mangun, & Gazzaniga, 1989 ; Pinto, Lamme, & de Haan, 2017b ; Volz, Hillyard, Miller, & Gazzaniga, 2018 ). It is important to note that even in this small group there are differences. In some patients all commissures were severed (“commissurotomy”), in others only the corpus callosum was cut (“callosotomy”) and some patients fall somewhere in between these two boundaries. Now, the search term “split-brain” results in a total of 2848 publications in the database of the Web-of-Science and 29,300 hits on Google Scholar, indicating a wealth of detailed information. The other depiction of split-brain patients entails the first-person perspective of the split-brain. In other words, “what is it like” to be a split-brain patient? It is especially this perspective that has captured the attention of the general press, popular science books and basic textbooks. By its nature, this second narrative lacks the detail of the functional description of the phenomenon, but it captures the intriguing question of how unity of consciousness is related to brain processes. Dominant in this description is the idea that in a split-brain each hemisphere houses an independent conscious agent. This notion, and particularly the concept of an isolated but conscious right hemisphere that is unable to express its feelings, desires or thinking due a lack of language, has captured the imagination (Gazzaniga, 2014 ).

It is important to clarify what we mean by unified consciousness. Here, we use the term in the sense of “subject unity” as defined by Bayne (Bayne & Chalmers, 2003 ; Bayne, 2008 ; Bayne, 2010 ). Subject unity is present if all the experiences generated in a system belong to one subject. In other words, if a system contains a first person perspective, then subject unity is preserved if that system only contains one such perspective, but subject unity is absent if the system contains multiple first person perspectives. Thus, in our definition of conscious unity, consciousness in a split-brain is split if each cortical hemisphere houses an independent conscious agent.

The view that consciousness is split in a split-brain has significantly impacted cognitive neuroscience at large. For instance, currently dominant theories about conscious awareness - the Integrated Information Theory (Tononi, 2005 ; Tononi, 2004 ) and the Global Neuronal Workspace Theory (Dehaene & Naccache, 2001 ; Dehaene, Kerszberg, & Changeux, 1998 ) - may be critically dependent on the validity of this view. Both theories imply that without massive communication between different subsystems, for instance cortical hemispheres, independent conscious agents arise. Thus, if the split consciousness view is invalid, these theories may be critically challenged.

The idea of split consciousness in a split-brain had its origin in the early split-brain studies (Gazzaniga, 1967 ; Gazzaniga, 1975 ; Gazzaniga et al., 1962 ; Sperry, 1968 ). These studies tested patients primarily in the two perceptual domains where processing is largely restricted to the contralateral hemisphere, that is vision and touch. In these early studies, stimuli, for instance objects, that were presented to the left hemisphere either physically in the right hand or as an image in the right visual half-field, could be readily named (as the left hemisphere is dominant for language) or pointed out with the right hand (which is controlled by the left hemisphere). The patient’s behavior became intriguing when the stimuli were presented in the left visual field or in the left hand. Now the patient, or at least the verbal left hemisphere, appeared oblivious to the fact that there had been a stimulus at all but was nevertheless able to select the correct object from an array of alternatives presented to the left hand or the left visual half-field (see Fig.  1 ). In a particularly dramatic recorded demonstration, the famous patient “Joe” was able to draw a cowboy hat with his left hand in response to the word “Texas” presented in his left visual half field. His commentary (produced by the verbal left hemisphere) showed a complete absence of insight into why his left hand had drawn this cowboy hat. Another astonishing example involved the same patient. MacKay and MacKay ( 1982 ) flashed a digit in the left visual field and trained the patient to play a version of ‘20 questions’ across hemispheres. The left hemisphere guessed the answer vocally, and the right hemisphere provided responses by pointing ‘up’ (meaning ‘guess a higher number’) or ‘down’ with the left hand. In this way the patient managed to vocalize the right answer. This suggests two independent conscious agents communicating with each other (one steering the left hand, the other agent controlling vocal expressions). However, note that an alternative interpretation is possible. Perhaps the patient knows the answer but finds it hard to vocalize. The ‘20 questions’ then simply help him in finding the correct vocalization.

One of the most well-known split-brain findings is that the patient claims verbally not to have seen the stimulus in the left visual field, yet indicates the identity of it with their left hand. This suggests that the left hemisphere (controlling verbal output) is blind to the left visual field, while the right hemisphere (controlling the left hand) does perceive it

Thus, these early observations suggested that there is no meaningful communication between the two hemispheres in split-brain patients. This led to the hypothesis that there might be two separate conscious agents, a left hemisphere that is able to talk to us and can explain what it sees and feels, and a mute right hemisphere that cannot communicate in language but that can nevertheless show that it has perceived and recognized objects and words. However, over time this view has eroded somewhat due to several anomalies (even right from the start) that may challenge this view.

Common Ground

An early observation, suggesting some remaining unification concerned what Joe Bogen called the “social ordinariness” of split-brain patients. Apart from a number of anecdotal incidents in the subacute phase following the surgery, these patients seem to behave in a socially ordinary manner and they report feeling unchanged after the operation (Bogen, Fisher, & Vogel, 1965 ; Pinto et al., 2017a ; R. W. Sperry, 1968 ; R. Sperry, 1984 ), although their extra-experiment behavior has not been systematically observed in great detail (Schechter, 2018 ). While the right hemisphere appears to be better at recognizing familiarity from faces, self-face recognition, that is the ability to realise immediately that a presented photograph represents you, appears to be available equally to both hemispheres in a split-brain patient (Uddin et al., 2008 ; Uddin, 2011 ). Thus, it seems unlikely that a mute but conscious right hemisphere would not have made itself known one way or the other. Thus, right from the start a paradox arose. The controlled lab tests suggested that consciousness is split in split-brain patients. Yet, everyday experiences of the patient and their close ones suggests that only one person exists in a split-brain. Additionally, it has been suggested that the two separate consciousnesses-hypothesis presumes that in the intact brain (before surgery) both hemispheres were conscious but connected via the corpus callosum, and they only became dissociable due to the operation. This casts doubt on the viability of the two consciousness view.

Crucially, the lab tests themselves were not always supportive of the split-consciousness view. Multiple experimental results showed that capacity for communication between the hemispheres varies both across patients and across tasks. For instance, a central observation in split-brain patients concerns the inability to compare visual stimuli across the midline. In other words, when one stimulus is presented to the left visual hemifield and the other to the right hemifield, the patient cannot accurately indicate whether both stimuli are the same or not, although they can do so when both stimuli are presented within one visual field. This is consistent with the notion that each hemisphere independently perceives the contralateral visual field, and that an intact corpus callosum is necessary for integration. Although there are indeed many examples of split-brain patients who are incapable of comparing stimuli across the midline, prominent examples can also be found of patients who can compare stimuli across the midline (Johnson, 1984 ; but see Seymour, Reuter-Lorenz, & Gazzaniga, 1994 ). This points to an important problem in the field, namely, individual differences. One aspect that may be important for individual differences is handedness differences. Variations in handedness may lead to differences in language capabilities in the right hemisphere, and could even underly differences in inter-hemispheric integration.

Moreover, under certain circumstances nearly all tested split-brain patients seem able to compare, or integrate, particular types of stimuli across the two visual half-fields (see Fig. 2 ). An early demonstration of across hemifields integration is the study by Eviatar and Zaidel ( 1994 ). They showed that split-brain patients could accurately indicate the identity and shape of upper- and lower-case letters in either hemifield, regardless of with which hand they responded, for instance accurately identifying the letter A in the left visual field with the right hand. Yet these patients were mostly unable to compare these same stimuli across visual fields. In another experiment, two tilted lines were presented with a gap between them. The lines were positioned in such a way that extending them across the gap would either cause the lines to coincide or to run in parallel. When split-brain patients indicate whether the lines are parallel or coincident, they are highly accurate, even when both line segments are located in different half-fields (Corballis, 1995 ; Pinto, de Haan, Lamme, & Fabri, n.d. ; Sergent, 1987 ; Trevarthen & Sperry, 1973 ). Another example of visual integration across the midline involves apparent motion. When two dots are presented in succession at a short distance (2 to 14 visual degrees), split-brain patients are able to accurately indicate whether the dots create apparent motion, or that they were presented simultaneously or with delays too long to provoke apparent motion. Critically, they are able to do so even when one dot appears in the left visual field, and the other in the right visual field (Knapen et al., 2012 ; Naikar & Corballis, 1996 ; Ramachandran, Cronin-Golomb, & Myers, 1986 ). Clearly, under specific conditions, there is meaningful communication between the two hemispheres in the absence of the corpus callosum.

Although most split-brain patients cannot compare visual features such as shape and object identity across the midline, other features, such as good continuation of lines, and apparent motion, are integrated without a corpus callosum

Another observation that suggests some form of unity across the two visual half fields concerns detection and localization of stimulation, for instance, a brief flash (see, for example, an early study on the response times to light flashes with the ipsi- or contralateral hand: Clarke & Zaidel, 1989 ). Several investigations (Corballis, Corballis, Fabri, Paggi, & Manzoni, 2005 ; Pinto et al., 2017a ; Trevarthen & Sperry, 1973 ) have demonstrated convincingly that split-brain patients can accurately report the presence and location of stimuli for any position in the whole visual field, with either hand, and even verbally. Accurate detection and localization appears to be possible for all patients and all stimuli (different shapes, figures, equiluminant stimuli) tested so far. Thus, when patients in earlier studies said that they saw “nothing” when a stimulus was presented in the left visual half-field, they may have meant that they could see it but that they could not identify or retrieve the name of the object.

Other findings point to a crucial difference between the degree of lateralization of visual-perceptual processing and producing overt responses. Perception appears to be more split, while responding remains largely unified. Whether a stimulus appears in the left or the right visual hemifield strongly impacts performance of split-brain patients. However, response type (left hand, right hand or verbally) seems to have a much smaller, or no effect at all. For instance, Pinto et al., 2017a ) found that the split-brain patient was much better at matching pictures to sample stimuli in the left visual field. Yet, for the exact same stimuli matching pictures to verbal labels was vastly superior when the stimuli appeared in the right visual field. Crucially, response type did not play any role. The patient was better in matching pictures to sample for stimuli in the left visual field, even if they responded verbally or with the right hand. Similarly, Levy, Trevarthen, and Sperry ( 1972 ) presented split-brain patients with chimeric or composite faces, that is, one half-face in each visual field. Subsequently the patient either matched the chimeric face to sample, or attached a verbal label to it. Verbal matching was mostly based on the half-face in the right visual field, while matching to sample was mainly driven by the half-face in the left visual field. But crucially, the latter was the case, independent of whether the patient responded with the left or the right hand.

Thus, it seems that in split-brain patients perceptual processing is largely split, yet response selection and action control appear to be unified under certain conditions. This, by itself, does not prove whether a split-brain houses one or two conscious agents. One explanation could be that the split-brain houses two agents, each having their own experiences, who synchronize their behavioral output through various means. Another possible explanation is that a split-brain houses one agent who experiences an unintegrated stream of information who controls the entire body, comparable to watching a movie where sight and sound are out-of-sync. At any rate, these findings challenge the previously mentioned classic split-brain description, which is still found in reviews and text books (Gray, 2002 ; Wolman, 2012 ). In this classic characterization the patient indicates that they saw nothing when a stimulus appeared in the left visual field. Yet, to their own verbal surprise, the left hand correctly draws the stimulus. The aforementioned examples of unity in action control suggests that these effects may depend on the type and complexity of the response that is required.

Interpretations

There are three, not-mutually exclusive, hypotheses concerning the mechanisms involved in, seemingly, preserved unity in the split-brain. The first notion is that information is transferred subcortically. The second idea is that ipsilateral motor control underlies unity in action control. The third idea claims that information transfer is based on varies forms of inter-hemispheric collaboration, including subtle behavioral cues. The first proposal (Corballis Corballis, Berlucchi, & Marzi, 2018 ; de Haan et al., 2019 ; Pinto, Lamme, & de Haan, 2017b ; Pinto et al., 2017a ; Savazzi et al., 2007 ; Mancuso, Uddin, Nani, Costa, & Cauda, 2019 ) suggests that the multitude of subcortical connections that are spared during surgery are responsible for the transfer of information. As was initially pointed out by Trevarthen ( 1968 ) and Trevarthen and Sperry ( 1973 ) and recently stressed by Pinto, de Haan, and Lamme ( 2017a ) and Corballis et al. ( 2018 ), there are many commissures (white matter tracts that connect homologous structures on both sides of the central nervous system) and decussations (bundles that connect different structures on both sides) that link nuclei that are known to be involved in perceptual processing. The importance of these commisural connections for transferring visual information in split-brain patients has been highlighted by Trevarthen and Sperry ( 1973 ). Moreover, the role of these connections in a split-brain has recently been demonstrated by bilateral fMRI activations in the first somatosensory cortex, after unilateral stimulation of trunk midline touch receptors (Fabri et al., 2006 ) and in the second somatic sensory area after unilateral stimulation of hand pain receptors (Fabri, Polonara, Quattrini, & Salvolini, 2002 ). Uddin and colleagues used low-frequency BOLD fMRI resting state imaging to investigate functional connectivity between the two hemispheres in a patient in whom all major cerebral commissures had been cut (Uddin et al., 2008 ). Compared to control subjects, the patient’s interhemispheric correlation scores fell within the normal range for at least two symmetrical regions. In addition, Nomi and colleagues suggested that split-brain patients might rely particularly on dorsal and ventral pontine decussations of the cortico-cerebellar interhemispheric pathways as evidenced by increased fractional anisotropy (FA) on diffusion weighted imaging (Nomi, Marshall, Zaidel, Biswal, Castellanos, Dick, Uddin & Mooshagian, 2019). Interhemispheric exchange of information also seems to occur in the domain of taste sensitivity, activation of primary gustatory cortex in the fronto-parietal operculum was reported in both hemispheres after unilateral gustatory stimulation of the tongue receptors (Mascioli, Berlucchi, Pierpaoli, Salvolini, Barbaresi, Fabri, & Polonara, 2015 ). Note that patients may differ with respect to how many of these connections have been cut, and this might also explain some of the individual variance among patients. Moreover, in all patients subcortical structures remain intact. For instance, the superior colliculus is known to integrate visual information from both hemispheres and project information to both hemispheres (Meredith & Stein, 1986 ; Comoli et al., 2003 ). Such structures may support attentional networks, and may enable the right hemisphere to attend to the entire visual field. In turn, attentional unity could help in unifying cognitive and motor control, which may subserve ipsilateral motor control.

The second point concerns the ipsilateral innervation of the arms. Manual action is not strictly lateralized, and the proximal (but not the distal) parts of the arm are controlled bilaterally, although the ipsilateral contribution remains undetermined. This could explain why split-brain patients may respond equally well with both hands in certain experimental conditions (Corballis, 1995 ; Gazzaniga, Bogen, & Sperry, 1967 ; Pinto, de Haan, & Lamme, 2017a ). First, there is substantial evidence that bilateral cortical activations can be observed during unilateral limb movements in healthy subjects. In addition, ipsilesional motor problems in arm control have been observed in patients with unilateral cortical injuries, and finally there is evidence from electrocorticography with implanted electrodes for localization of epileptic foci showing similar spatial and spectral encoding of contralateral and ipsilateral limb kinematics (Bundy, Szrama, Pahwa, & Leuthardt, 2018 ). While these observations argue convincingly for a role in action control by the ipsilateral hemisphere, they do not prove that a hemisphere on it’s own can purposefully control the movements of the ipsilateral hand. Thus, the role of ipsilateral arm-hand control in explaining split-brain findings is currently not settled.

The third hypothesis argues that in addition to whatever direct neural communication may exist between the hemispheres, they may inform one another via strategic cross-cueing processes (Volz & Gazzaniga, 2017 ; Volz et al., 2018 ). The split-brain patients underwent surgery many years prior to testing, and the separated perceptual systems have had ample time to learn how to compensate for the lack of commissural connections. For example, subtle cues may be given by minimal movements of the eyes or facial muscles, which might not even be visible to an external observer but are capable of encoding, for example, the location of a stimulus for the hemisphere that did not “see” it. A cross-cueing mechanism might also allow one hemisphere to convey to the other which one of a limited set of known items had been shown (Gazzaniga & Hillyard, 1971 ; Gazzaniga, 2013 ).

Finally, it is possible to entertain combinations of the different explanations. For instance, it is conceivable that in the subacute phase following split-brain surgery the hemispheres are ineffective in communicating with each other. During this initial phase, phenomena such as an “alien hand” - that is a hand moving outside conscious control of the (verbal) person - may be present. In the ensuing period, the patients may have learned to utilize the information that is exchanged via subcortical connections, ipsilateral motor control or cross-cueing to coordinate the processing of the two hemispheres. In such a way, the patient may counteract some of the effects of losing the corpus callosum.

What do We Need to Know?

This paper aims to contribute to the agenda for the next decade of split-brain research. Full split-brain surgery is rare these days, and it is important that we try to answer the central questions while these patients are still available for study. In order to examine the variations between patients it would be useful to test as many of the available patients as possible with the same tests.

One important goal is to map out precisely how much functionality and information is still integrated across hemispheres in the split-brain, and what the underlying principles are. For instance, in some cases the two hemispheres seem to carry out sensory-motor tasks, such as visual search, independently from one another (Arguin et al., 2000 ; Franz, Eliassen, Ivry, & Gazzaniga, 1996 ; Hazeltine, Weinstein, & Ivry, 2008 ; Luck, Hillyard, Mangun, & Gazzaniga, 1994 ; Luck et al., 1989 ), while in other cases functions such as attentional blink, or attentional cueing, seem to be integrated across hemispheres (Giesbrecht & Kingstone, 2004 ; Holtzman, Volpe, & Gazzaniga, 1984 ; Holtzman, Sidtis, Volpe, Wilson, & Gazzaniga, 1981 ; Pashler et al., 1994 ; Ptito, Brisson, Dell’Acqua, Lassonde, & Jolicœur, 2009 ). An important challenge is to unveil why some cognitive functions can be carried out independently in the separated hemispheres while other functions engage both hemispheres. Furthermore, it is now clear that accurate detection and localization is possible across the whole visual field, and there is some evidence that even more information concerning visual images can be transferred between hemispheres. Although we have some understanding of what types of information can be transferred in the visual domain, our knowledge base in the somatosensory domain is much more limited. This is probably due to a bias throughout cognitive neuroscience and psychology, leading to a strong focus on vision in split-brain research. It is important to collect converging evidence by investigating the somatosensory system which is also strongly lateralized. Note that in somatosensory processing transfer between hemispheres (about 80% correct for the bimanual conditions) has been observed for basic same-different matching of real objects (Fabri, Del Pesce et al., 2005 ).

Another important goal is to obtain a more detailed description of the perceptual, cognitive and linguistic capabilities of the disconnected right hemisphere. For understanding unity of mind, two capabilities specifically are crucial. First, experiments investigating aspects of the conscious mind often go beyond simple visual processing, and future studies will thus critically depend on testing high-level cognitive abilities of both hemispheres. Specifically, language abilities, crucial for understanding questions and instructions, will likely play a pivotal role. Thus, the first question is to what extent the right hemisphere is capable of language processing. Note that complicated instructions (Gazzaniga, Smylie, Baynes, Hirst, & McCleary, 1984 ; Pinto et al., 2017a ; Zaidel, 1983 ), for instance relating to mental imagery (Johnson, Corballis, & Gazzaniga, 2001 ; Kosslyn, Holtzman, Farah, & Gazzaniga, 1985 ; Sergent & Corballis, 1990 ), seem to be well within the reach of the right hemisphere. Moreover, right hemisphere language capabilities seem to improve over time (Gazzaniga, Volpe, Smylie, Wilson, & LeDoux, 1979 ; Gazzaniga et al., 1996 ). Longitudinal language tests (for instance with a Token test: De Renzi & Vignolo, 1962 ) would further illuminate the extent of right hemisphere language processing.

Second, unveiling to what extent each hemisphere is capable of subserving consciousness at all seems relevant for unity of mind as well. If the disconnected right hemisphere can produce full-blown consciousness, then questions regarding unity of mind are clearly more pertinent then if the right hemisphere only produces minimal amounts of consciousness. Right hemisphere consciousness can be studied through novel neural paradigms (Bekinschtein et al., 2009 ; Casali et al., 2013 ; Pitts, Metzler, & Hillyard, 2014 ; Shafto & Pitts, 2015 ). For instance, Bekinschtein et al. employed EEG to measure if the brain detected irregularities (as indicated by an event-related potential [ERP] signal called the P3) in different states of consciousness. They found that when consciousness was reduced, local irregularities were still detected - for instance after three high auditory tones a low tone evoked a P3. However, global irregularities - several times a low tone followed three high tones, then on the critical trial three high tones were followed by another high tone - did not evoke a P3 when consciousness was reduced. Crucially, when consciousness was unimpaired both local and global irregularities evoked a P3 response. Right hemisphere consciousness may also be studied in other patient groups where interhemispheric communication is hampered. One particularly interesting group are post-hemispherotomy patients (Lew, 2014 ). These patients have been surgically treated to disconnect an entire hemisphere (usually for intractable epilepsy), but unlike hemispherectomy patients the disconnected hemisphere remains in place in the cranium and remains vascularized.

Clearly, the central question, whether each hemisphere supports an independent conscious agent, is not settled yet. Novel paradigms in this respect could lead to progress. For instance, a pivotal question is whether each hemisphere makes its own decisions independent of the other hemisphere. If each hemisphere produces its own autonomous conscious agent then this should be the case. That is, if two agents are asked to freely choose a random number, then the odds that they consistently pick the same number are small. And vice versa, if each hemisphere makes its own conscious decisions, independent of the other hemisphere, then this seems to rule out unity of mind. Note that each hemisphere making its own decisions is different from information processing occurring independently per hemisphere. Unconscious information processing is almost certainly split across hemispheres in a split-brain. However, this does not prove that consciousness is split or unified. Even in a healthy brain, where consciousness is unified, many unconscious processes run independently, and in parallel.

One way to tackle the central question is by having the hemispheres respond to questions in parallel. Overt behavior most likely does not allow for this, due to bilateral motor control processes sketched earlier. However, perhaps parallel responding is possible if the hemispheres produce covert responses. For instance, the patient could be asked to pick one of four options and indicate their choice by carrying out certain content-specific mental imagery tasks. This imagery can then be decoded in parallel from each hemisphere using neuroimaging techniques (see Owen et al., 2006 for a similar approach with vegetative state patients). If each hemisphere harbors an autonomous conscious agent, then it is highly unlikely that the two hemispheres will consistently make the same choices. Thus, if the choices are uncorrelated across hemispheres, then this may critically challenge the unified mind view.

Another way to tackle the question of unified consciousness in the split-brain is to employ ERPs as markers of concurrent conscious processing in the left and right hemispheres. For instance, in one study (Kutas, Hillyard, Volpe, & Gazzaniga, 1990 ) visual targets were presented either separately to the left or right visual field or to both visual fields simultaneously. It was found that the P300 - a signal possibly reflecting conscious processing of a visual target (Dehaene & Changeux, 2011 ; Dehaene, Charles, King, & Marti, 2014 ; Salti, Bar-Haim, & Lamy, 2012 ) - was reduced for bilateral targets. This suggests some type of integration of conscious processing. Studies employing ERPs may indicate whether conscious processing is unified, while unconscious processing is split, which would be suggestive of unified consciousness.

Conclusions

In summary, the pivotal issue in split-brain research is whether dividing the brain divides consciousness. That is, do we find evidence for the existence of one, or two conscious agents in a split-brain? Note that intermediate results may be found. Perhaps some measures indicate unified consciousness while others do not. This would then provoke further interesting questions on the unity of consciousness. What are the crucial measures for unity of consciousness? If intermediate results are found, more unconventional possibilities should be entertained as well. For instance, although difficult to fathom, some philosophers have suggested that a split-brain does not contain one or two observers, but a non-whole number of conscious agents (Nagel, 1971 ; Perry, 2009 ), for instance one and a half first-person perspectives. If evidence for this position is found, then its implications would stretch beyond split-brain patients. It would suggest that our intuitions on the indivisibility of the experiential self may be mistaken. One way to think of this is as with the difference between conscious and unconscious processing. Perhaps this is not a dichotomous distinction, but a continuum between more or less conscious. Similarly, perhaps the existence of a first-person perspective is not dichotomous, but gradual as well. Another possibility is that a split-brain does contain a whole number of conscious agents, but that consciousness across these agents is only partially unified. That is, the agents share some conscious experiences and decisions, but not all (Lockwood, 1989 ; Schechter, 2014 ; Schechter, 2018 ; Schechter, 2013 ). Finally, another way to look at this is in terms of ‘dissociation’, as in depersonalization (Phillips et al., 2001 ; Sierra et al., 2002 ). Perhaps the number of agents is not altered, but the agent feels depersonalized in some situations, and therefore no longer feels that they control the actions, or even experience the information, that has just occurred in their brain.

New findings on the unity of consciousness in a split-brain could fundamentally impact currently dominant consciousness theories. Global Neuronal Workspace Theory (Dehaene & Naccache, 2001 ; Dehaene et al., 1998 ) asserts that consciousness arises when information that is processed in unconscious (or preconscious) modules is broadcast to a central ‘workspace’, primarily residing in frontal regions of the brain. Although not very explicit on the unity of consciousness in split-brain patients, Global Neuronal Workspace Theory seems to endorse the split consciousness idea, given that each hemisphere has its own prefrontal hub, enabling broadcasting of whatever information is processed in that hemisphere.

Integrated Information Theory (Tononi, 2005 ; Tononi, 2004 ) has specifically addressed the issue of split brain (for instance in Tononi, 2004 ). Integrated Information Theory asserts that ‘phi’, a measure of how integrated information is, determines the level of consciousness. The higher phi, the more conscious a system is. Moreover, local maxima in phi correspond to conscious agents. If in a system all subsystems are highly interconnected, then phi is highest for the system as a whole, and local maxima are absent. Thus, such a system produces only one conscious agent. However, if subsystems only exchange minimal amounts of information, then phi per subsystem is higher than phi for the system as whole. In such a case each subsystem creates its own conscious agent. In a split-brain, connectedness, that is integration of information, is much higher within than across hemispheres. Therefore, according to Integrated Information Theory consciousness should be split in a split-brain.

Recurrent Processing theory (Lamme & Roelfsema, 2000 ; Lamme, 2004 ; Lamme, 2010 ) argues for the independence of consciousness from attention, access, or report. This theory has addressed the issue of report specifically, making the case that consciousness and reportability, whether verbal or manual, should be viewed as entirely independent (Lamme, 2006 ; Tsuchiya, Wilke, Frässle, & Lamme, 2015 ). Crucially, this theory states that even in the normal mind, ‘islands’ of unattended yet conscious information reside (Lamme, 2006 ). In these cases, all the information, although functionally unintegrated, is nonetheless experienced by the same mind. Support for this view comes from findings in multiple object tracking (Pinto, Howe, Cohen, & Horowitz, 2010 ; Pinto, Scholte, & Lamme, 2012 ; Pylyshyn & Storm, 1988 ). Here, evidence indicates that when moving objects in two visual hemifields are tracked, attention is split (Howe, Cohen, Pinto, & Horowitz, 2010 ) and each hemisphere processes the relevant information in the contralateral visual field independently of the other (Alvarez & Cavanagh, 2005 ; Cavanagh & Alvarez, 2005 ; Drew, Mance, Horowitz, Wolfe, & Vogel, 2014 ). That is, the left hemisphere only tracks the right visual field and vice versa. Yet, although the visual information is not integrated across hemispheres, from a first person perspective, it seems clear that the subject experiences all moving objects across the entire visual field. Another example of the dissociation between consciousness and reportability is the so-called partial report paradigm (Pinto et al., 2017b ; Pinto, Sligte, Shapiro, & Lamme, 2013 ; Sligte, Scholte, & Lamme, 2008 ; Sperling, 1960 ). In these paradigms subjects seem to remember more than they can report. Thus, reportability and consciousness are dissociated. Perhaps in split-brain patients this dissociation is simply more pronounced. That is, consciousness remains unified, but reportability has become more dissociated, thereby inducing the appearance of two independent agents. In sum, according to the Recurrent Processing theory, integration of information is not needed for a unified mind, implying that the mind may remain unified when the brain is split. Thus, different theories of consciousness have different predictions on the unity of mind in split-brain patients, and await the results of further investigation into this intriguing phenomenon.

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This work was supported by an Advanced Investigator Grant by the European Research Council (ERC grant FAB4V (#339374) to EdH and a Templeton grant (ID# 61382, "Towards understanding a unified mind") to YP and VL.

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de Haan, E.H.F., Corballis, P.M., Hillyard, S.A. et al. Split-Brain: What We Know Now and Why This is Important for Understanding Consciousness. Neuropsychol Rev 30 , 224–233 (2020). https://doi.org/10.1007/s11065-020-09439-3

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• Published: 01 August 2005

Forty-five years of split-brain research and still going strong

• Michael S. Gazzaniga 1  

Nature Reviews Neuroscience volume  6 ,  pages 653–659 ( 2005 ) Cite this article

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Forty-five years ago, Roger Sperry, Joseph Bogen and I embarked on what are now known as the modern split-brain studies. These experiments opened up new frontiers in brain research and gave rise to much of what we know about hemispheric specialization and integration. The latest developments in split-brain research build on the groundwork laid by those early studies. Split-brain methodology, on its own and in conjunction with neuroimaging, has yielded insights into the remarkable regional specificity of the corpus callosum as well as into the integrative role of the callosum in the perception of causality and in our perception of an integrated sense of self.

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Acknowledgements

This research was supported by National Institutes of Health grants to the author. It was also supported by a graduate reseach fellowship from the National Science Foundation to M. Colvin. I would like to thank my collaborators, M. Colvin, M. Funnell, M. Roser and D. Turk, for their scientific input as well as their assistance in reviewing this paper. I would also like to thank R. Townsend for her editorial assistance.

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Gazzaniga, M. Forty-five years of split-brain research and still going strong. Nat Rev Neurosci 6 , 653–659 (2005). https://doi.org/10.1038/nrn1723

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Split brain: divided perception but undivided consciousness

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Yair Pinto, David A. Neville, Marte Otten, Paul M. Corballis, Victor A. F. Lamme, Edward H. F de Haan, Nicoletta Foschi, Mara Fabri, Split brain: divided perception but undivided consciousness, Brain , Volume 140, Issue 5, May 2017, Pages 1231–1237, https://doi.org/10.1093/brain/aww358

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In extensive studies with two split-brain patients we replicate the standard finding that stimuli cannot be compared across visual half-fields, indicating that each hemisphere processes information independently of the other. Yet, crucially, we show that the canonical textbook findings that a split-brain patient can only respond to stimuli in the left visual half-field with the left hand, and to stimuli in the right visual half-field with the right hand and verbally, are not universally true. Across a wide variety of tasks, split-brain patients with a complete and radiologically confirmed transection of the corpus callosum showed full awareness of presence, and well above chance-level recognition of location, orientation and identity of stimuli throughout the entire visual field, irrespective of response type (left hand, right hand, or verbally). Crucially, we used confidence ratings to assess conscious awareness. This revealed that also on high confidence trials, indicative of conscious perception, response type did not affect performance. These findings suggest that severing the cortical connections between hemispheres splits visual perception, but does not create two independent conscious perceivers within one brain.

A depiction of the traditional view of the split brain syndrome ( top ) versus what we actually found in two split-brain patients across a wide variety of tasks ( bottom ). The canonical idea of split-brain patients is that they cannot compare stimuli across visual half-fields ( left ), because visual processing is not integrated across hemispheres. This is what we found as well. However, another key element of the traditional view is that split-brain patients can only respond accurately to stimuli in the left visual field with their left hand and to stimuli in the right visual field with their right hand and verbally. This is not what we found. Across a wide variety of tasks, we observed that split-brain patients could reliably indicate presence, location, orientation and identity of stimuli throughout the entire visual field regardless of how they responded.

Strikingly, although this clinical observation features in many textbooks ( Gazzaniga et al. , 1998 ; Gray, 2002 ) the reported data are never quantitative. For three reasons it is important to explicitly map out how often ‘blindness’ to the left visual field is indicated by verbal/right hand responses and unawareness to the right visual field is indicated by left hand responses. First, the number of split-brain patients is now rapidly decreasing, and it will soon be impossible to study this phenomenon. Second, there is some doubt about how clear-cut the textbook findings are. In one of the seminal publications on this topic, Sperry (1968) reports that split-brain patients seem blind to the left visual field when responding with the right hand and vice versa. However, in the last paragraph (p. 733), Sperry notes: ‘Although the general picture has continued to hold up in the main as described [… .] striking modifications and even outright exceptions can be found among the small group of patients examined to date’. Moreover, Levy et al. (1972) investigated perception of chimeric faces in five split-brain patients. Although not the focus of their research, they observed that all patients were better at matching a face to a sample when the face was presented in the left visual field, regardless of whether they responded with the left or the right hand (p. 65). Finally, note that there are multiple examples in the literature suggesting some kind of interhemispheric integration of information ( Corballis and Trudel, 1993 ; Corballis, 1995 ; Corballis and Corballis, 2001 ; Savazzi and Marzi, 2004 ; Savazzi et al. , 2007 ). This, like Sperry’s (1968) closing remark, casts doubt on the precise nature of the split-brain phenomenon.

Third, the status of split-brain patients may have important consequences for current dominant theories of consciousness. Congruent with the canonical view of split-brain patients, both the Global Workspace theory ( Baars, 1988 , 2005 ; Dehaene and Naccache, 2001 ) and the Information Integration theory ( Tononi, 2004 , 2005 ; Tononi and Koch, 2015 ) imply that without massive interhemispheric communication two independent conscious systems appear. If the canonical view cannot be quantitatively replicated, and evidence for conscious unity in the split-brain syndrome is found, both theories may require substantial modifications.

In our current studies we reproduced the classic finding that split-brain patients are unable to integrate visual information across the two visual half-fields. However, we also investigated systematically to what extent performance depends on where a stimulus appears. For various tasks and stimuli we studied whether there is a response type × visual field interaction: can split-brain patients only respond to stimuli in the left visual field with the left hand, and to stimuli in the right visual field with the right hand or verbally?

Patients were tested across several years, during their routine neurological control visits. For Experiment 1 we tested Patients DDC and DDV, for Experiments 2–5 we tested Patient DDC. Both patients underwent a full callosotomy to relieve epileptic seizures. Crucially, for current purposes, in Patient DDC the complete corpus callosum and most of the anterior commissure was cut, and in Patient DDV the complete corpus callosum was removed. We selected Patient DDC for the extensive follow-up testing since his ‘split’ is the most severe. Note that other than the removal of the corpus callosum, both patients had no brain damage, and fell within the normal IQ range. See Supplementary material and Pizzini et al. (2010) and Corballis et al. (2010) for detailed descriptions of these patients. In all experiments the patient(s) responded with three response types (response conditions were blocked), verbal, right hand or left hand, except for Experiment 2A, where the patient only responded verbally; and Experiments 2C and 4A where only left and right hand responses and no verbal responses were given. The experimenter (who could not see the test stimuli) mouse-clicked on the response box or location indicated by the patient. In the case of verbal position indication, the mouse was moved by the experimenter (not having seen the stimulus) on the instructions of the patient until the desired position was obtained.

In Experiment 1 both patients performed a combined detection/localization task. Either nothing appeared (50% of trials) or a red solid circle, on a grey background (see Supplementary material for all stimulus details), appeared for 120 ms anywhere in the visual field. Each trial the patient indicated whether a stimulus had appeared, and if so where.

In Experiment 2A, Patient DDC indicated whether two rectangles had the same orientation. In Experiment 2B he reported if two simple shapes were the same, and in Experiment 2C he indicated if two pictures were equal. In all experiments the test stimuli appeared for 120 ms. The stimuli appeared (i) both in left visual field; (ii) both in right visual field; or (iii) they appeared around fixation with one stimulus in left visual field and on in right visual field. In Experiment 3A a picture was presented for 120 ms in the left or right visual field, after which Patient DDC selected the correct verbal label matching the picture. Experiment 3B was identical to 3A, but instead of selecting a verbal label, Patient DDC selected from two pictures which image he had just seen.

In Experiment 4A either nothing appeared, or a simple shape (square, circle or triangle) appeared for 100 ms in the left or right visual field. Patient DDC indicated if something had appeared, and if so what. In Experiment 4B two rectangles were successively presented, the first of which appeared for 120 ms, in the left or right visual field. Patient DDC indicated whether both rectangles had the same orientation, and if not, how large the orientation difference was. In both experiments, after each trial, Patient DDC indicated confidence in his judgement (Experiment 4A on a scale from 1 to 4, Experiment 4B on a scale from 1 to 4). Experiment 5 was similar to Experiment 1, except after each trial Patient DDC indicated confidence in his presence and location judgement (on a scale from 1 to 5). Moreover, stimuli were bright green on a red background, or dim green on a red background. In the latter case stimuli and background were equiluminant (as determined by an objective measurement).

An overview of the results of Experiment 1. Both split-brain patients, Patients DDC and DDV, accurately indicated presence and location (distance error is in degrees of visual angle) of stimuli appearing throughout the entire visual field, regardless of response type (verbally, left hand or right hand). These findings challenge the canonical view that split-brain patients can only respond correctly to the left visual field with the left hand and vice versa.

In Experiment 1 ( Fig. 2 ), we explored to what extent Patients DDV and DDC can detect stimuli across the entire visual field using three response conditions: left hand, right hand, and verbally. Subjects were shown red circles in various locations of the visual field (50% of trials no stimulus was presented), and had to detect presence or absence either verbally or by indicating yes/no with either hand. Subsequently, for seen stimuli, they had to indicate the location of the stimulus. Both patients responded (nearly) perfectly in indicating presence of the stimulus (Patient DDV, hits: 100%, false alarms: 0%; Patient DDC, hits: 97.5%, false alarms: 7.7%), and were highly accurate in indicating location of the stimulus (average distance between pointed location and actual location: Patient DDV: 2.8°, Patient DDC: 4.5°). While presence and location performance was highly significantly above chance (all P < 0.001), the response type × visual half-field interaction did not approach significance in either patient or task (all P > 0.5).

An overview of the results of Experiments 2 and 3. Patient DDC was not able to compare stimuli across visual half-fields, although he was able to do so within one visual half-field (Experiment 2A–C). Moreover, he was better at labelling stimuli in the right visual field (Experiment 3A) and better at matching stimuli in the left visual field (Experiment 3B). Crucially, although visual information remained unintegrated across visual half-fields, there was still no response type × visual field interaction.

We found further evidence that visual information is not shared between hemispheres in Experiment 3 ( Fig. 3 ). Here we observed that Patient DDC was better at selecting the correct verbal label for an image when it had appeared in the right visual field than when it had appeared in the left visual field (Experiment 3A, left visual field: 73.4%, right visual field: 92.1%, left visual field versus right visual field: P < 0.001). Yet, he was better at matching a stimulus to sample for items in left visual field, replicating earlier split-brain findings ( Funnell et al. , 1999 ) (Experiment 3B, left visual field: 95.5%, right visual field: 73%, left visual field versus right visual field: P < 0.001). Note that, despite the seeming lack of transfer of visual information, we still observed no response type × visual field interaction in Experiments 2 and 3 (all P > 0.12). Thus, for instance, Patient DDC was better at matching to sample of stimuli in the left visual field even when he responded with the right hand. This suggests that processing of visual stimuli remains within each individual hemisphere, each with its own relative performances in various tasks, yet control over the report of the outcomes of this processing is undivided.

Across three experiments Patient DDC performed better on high confidence than on low confidence trials, suggesting accurate metacognition. Moreover, also for high confidence trials we observed no response type × visual field interaction, suggesting that unity in responding was based on conscious perception, not on blindsight-like processes.

First, Patient DDC was tested on two visual matching experiments (shape and orientation). Second, he performed a detection and localization task of simple stimuli as in the first experiment (with the addition that the stimuli were presented equiluminantly with the background or with a large luminance difference). Patient DDC performed nearly flawlessly in detecting objects in Experiment 5 (no false alarms and two misses in 167 trials). This ceiling effect precluded meaningful metacognitive assessment of this aspect of the task. However, in the other two experiments and the localization of objects in Experiment 5, performance did not show a ceiling or floor effect, allowing us to investigate metacognitive abilities in these cases. This revealed that in all three experiments Patient DDC’s performance was better on high than on low confidence trials. All trials: Experiment 4A, left visual field: 88.7%, right visual field: 43%; Experiment 4B, left visual field: 82.8% right visual field: 63.4%; Experiment 5, left visual field: accuracy: 100%, distance error: 3.27°, right visual field: accuracy: 98.3%, distance error: 2.33°. High confidence trials: Experiment 4A, left visual field: 100%, right visual field: 62.5%; Experiment 4B, left visual field: 95.9% right visual field: 84.6%; Experiment 5, left visual field: accuracy: 100%, distance error: 2.63°, right visual field: accuracy: 98.2%, distance error: 1.84°; all P < 0.005). Further, we found a robust Goodman and Kruskal’s γ correlation ( Goodman and Kruskal, 1954 ) between confidence and performance in all cases (Experiment 4A, γ = 0.527, P < 0.001; Experiment 4B, γ = 0.316, P = 0.003; Experiment 5, γ = −0.227, P = 0.02. There were no differences between γ correlations in left visual field and right visual field, all P > 0.09). Both analyses indicate that Patient DDC possessed accurate metacognition. Crucially, on high confidence trials we still found no response type × visual field interaction (all P > 0.63). This indicates that Patient DDC’s performance is not rooted in unconscious processes: his correct answers are based on conscious awareness and decisions. Note further that in the detection and localization task, luminance difference did not affect results (all P > 0.8), indicating that our findings are not due to overly strong stimulation, or stray-light leaking over to the other visual half-field.

In addition to these five experiments we obtained phenomenal reports from both split-brain patients (see Supplementary material for an extensive description). Both patients indicated that they saw their entire visual field (so not just the visual field to the left or right of fixation). Further, they indicated that they felt, and were in control of their entire body. Finally, they reported that their conscious unity was unchanged since the operation (i.e. no other conscious agent seemed to be present in their brain/body). These phenomenal reports are congruent with earlier reports of split-brain patients, which documented that split-brain patients feel normal and behave normally in social situations ( Bogen et al. , 1965 ; Sperry, 1968 ).

In conclusion, with two patients, and across a wide variety of tasks we have shown that severing the cortical connections between the two hemispheres does not seem to lead to two independent conscious agents within one brain. Instead, we observed that patients without a corpus callosum were able to respond accurately to stimuli appearing anywhere in the visual field, regardless of whether they responded verbally, with the left or the right hand—despite not being able to compare stimuli between visual half-fields, and despite finding separate levels of performance in each visual half-field for labelling or matching stimuli. This raises the intriguing possibility that even without massive communication between the cerebral hemispheres, and thus increased modularity, unity in consciousness and responding is largely preserved.

This preserved unity of consciousness may be especially challenging for the two currently most dominant theories of consciousness, the Global Workspace theory ( Baars, 1988 , 2005 ; Dehaene and Naccache, 2001 ) and the Integration Information theory ( Tononi, 2004 , 2005 ; Tononi and Koch, 2015 ). A core assumption of the Global Workspace theory is that cortical broadcasting of selected information by the ‘global workspace’ leads to consciousness. Thus severing of the corpus callosum, which prevents broadcasting of information across hemispheres, seems to exclude the emergence of one global workspace for both hemispheres. Rather, it seems that without a corpus callosum either two independent global workspaces emerge, or only one hemisphere will have a global workspace, while the other does not. In either case, an integrated global workspace, and thus preserved conscious unity, seems to be difficult to fit into this framework.

Also for Integration Information theory, conscious unity in the split-brain syndrome seems to be challenging. According to the Integration Information theory the richness of integration of information (called φ, defined by how much information is represented, and how integrated the information is) determines the level of consciousness. Moreover, only if the combined φ of two subsystems is larger than the φ per system, then the two subsystems combine to form one conscious entity. After removal of the corpus callosum, which all but eliminates communication between the cerebral hemispheres, integration of information is larger within each hemisphere than between hemispheres. Thus, according to the Integration Information theory, in the split-brain syndrome φ per hemisphere is larger than the combined φ, thus leading to two independent conscious systems rather than one conscious agent ( Tononi, 2005 ).

It thus seems that the current results provide a challenge for the Global Workspace and the Integrated Information theory of consciousness. However, the current results may fit well with the local recurrent processing theory of consciousness ( Lamme and Roelfsema, 2000 ; Lamme, 2006 ; Block, 2007 ). This theory claims that local recurrent interactions between neural areas (for example between V1 and V5 in the visual system) are enough to create consciousness, even if these interactions are not part of a larger integrated network, and do not project their outcomes to a central processing unit. Thus, according to this theory, even in healthy subjects, relatively isolated processing in one hemisphere can lead to normal visual experiences. Therefore, the local recurrent processing theory suggests that consciousness in split-brain patients may be similar to consciousness in healthy subjects (and thus equally unified).

How should these results be compared to our classic text-book knowledge of the split-brain phenomenon? It is unlikely that our results can be explained by the anterior and posterior commissure still being (somewhat) intact, as this was also the case for many of the previously tested patients, and this did not seem to play an important role then ( Gazzaniga et al. , 1985 ; Seymour et al. , 1994 ; Gazzaniga, 2005 ).

Another possible explanation to consider is that the current findings were caused by cross-cueing (one hemisphere informing the other hemisphere with behavioural tricks, such as touching the left hand with the right hand). We deem this explanation implausible for four reasons. First, cross-cueing is thought to only allow the transfer of one bit of information ( Baynes et al. , 1995 ). Yet, both patients could localize stimuli throughout the entire visual field irrespective of response mode (Experiments 1 and 5), and localizing a stimulus requires more than one bit of information. Second, visual capabilities differed per hemifield (Experiment 3: better matching for stimuli in left visual field, better labelling of stimuli in right visual field) and comparison of stimuli over hemifields was not possible (Experiment 2). This suggests that transfer of visual information did not occur. Yet, in these same experiments response type did not affect performance, suggesting that unity in control was not driven by any form of transfer of visual information. Third, we explicitly set up the experiments to prevent cross-cueing (e.g. hands were not allowed to touch each other, or the other half of the body). Moreover, we did not observe any indications of cross-cueing occurring. Fourth, as cross-cueing is a slow process, ipsilateral responses driven by cross-cueing should be considerably slower than contralateral responses. Yet, in one experiment where Patient DDC indicated, as quickly as possible, the colour of a circle appearing shortly to the left or the right of fixation, average ipsilateral and contralateral responses were almost equally fast, and equally accurate (ipsilateral reaction times: 1229 ms, ipsilateral accuracy: 88.4%; contralateral reaction times: 1307 ms, contralateral accuracy: 97%; No significant difference between ipsilateral and contralateral reaction times: P = 0.13; or between ipsilateral and contralateral accuracy: P = 0.55, see Supplementary material for details).

Finally, a possibility is that we observed the current results because we tested these patients well after their surgical removal of the corpus callosum (Patient DDC and Patient DDV were operated on at ages 19 and 22 years, and were tested 10–16 and 17–23 years after the operation, respectively). This would raise the interesting possibility that the original split brain phenomenon is transient, and that patients somehow develop mechanisms or even structural connections to re-integrate information across the hemispheres, particularly when operated at early adulthood. Even then, it remains the case that these patients’ minds have a curious property: somehow, their perception seems split, each hemisphere processing visual information independently, and at the best of their individual—yet different—abilities. When it comes to reporting this information to the outside world, however, the outcomes of the perceptual processes are unified in consciousness, verbalization and control of the body. This ‘split phenomenality’ combined with ‘unity of consciousness’ is difficult to grasp introspectively, and surely warrants further study, in a group of patients of which very few remain today.

We thank Gabriella Venanzi for scheduling the patients’ exams. We also thank D.D.C. and D.D.V. and their families for their willingness to collaborate in these studies.

This research was supported by a Marie Curie IEF grant (PIEF-GA-2011-300184) to Y.P., a Marie Curie IEF grant (PIEF-GA-2012-SOC-329134) to M.O. and by Advanced Investigator Grants by the European Research Council to V.A.F.L. and E.H.FdH.

Supplementary material

Supplementary material is available at Brain online.

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The Life and Contributions of Roger Sperry: Exploring the Discovery of Left and Right Hemisphere

If you have ever heard of right and left brain hemispheres , then you have Roger Sperry to thank for it. This Nobel Prize-winning neuropsychologist is one of the most significant figures in the field of psychology. Sperry made significant breakthroughs in the research and understanding of the brain’s functionality that led to a better comprehension of human behavior and cognitive abilities .

In this blog post, we will delve into who Roger Sperry was, his life and achievements, his contribution to psychology, and his famous Split-Brain Experiment.

Who was Roger Sperry?

Roger Sperry was born on August 20, 1913, in Hartford, Connecticut. He was an American neuropsychologist, neurobiologist, and Nobel laureate who was known for his groundbreaking work in split-brain research. Sperry graduated from Oberlin College with a bachelor’s degree in English in 1935 before enrolling at the University of Chicago for his graduate studies.

He obtained his doctorate degree from the University of Chicago in 1941. Sperry then served at the California Institute of Technology (Caltech) from 1953 to 1984, where he conducted his most renowned research and experiments, earning him numerous accolades and distinctions.

What is Roger Sperry best known for?

Sperry is famous for his work on the split-brain hemisphere, the concept of lateralization of brain function, and neuropsychology. Through his studies, he demonstrated that the two hemispheres of the brain process information differently and can be separated surgically, which led to his Nobel Prize recognition.

Sperry’s researches showed that each hemisphere of the brain has specialized functions. The left hemisphere is responsible for logical, analytical, and verbal tasks, while the right hemisphere is more visual, orientation, and spatial tasks. He also contributed to our understanding of the brain’s plasticity, how it adapts and changes to different stimuli, and the limitations of brain function when injured.

Where did Roger Sperry work?

After Sperry completed his studies, he became an instructor at Harvard Medical School before joining the faculty of the University of Chicago. In 1953, he took up a position at the California Institute of Technology, where he worked until his retirement in 1984.

During his time at Caltech, Sperry worked closely with Michael Gazzaniga, who would later carry on his research into split-brain patients . Their work together would go on to make major contributions to our understanding of the mind and the brain.

Michael Gazzaniga Contribution to Psychology

Michael Gazzaniga, a student of Sperry, carried on his research into split-brain patients and made significant contributions to the understanding of the mind and brain. Gazzaniga’s research on the left brain interpreter, which is responsible for making sense of events in our lives, expanded and built on Sperry’s work.

He showed that the left hemisphere tries to make sense of information, even if it’s false, incorrect, or doesn’t make any sense. This research has influenced our understanding of cognitive dissonances and how humans make judgments and decisions.

What did Roger Sperry discover?

Roger Sperry’s most significant discovery was that the two hemispheres of the brain, the left and right hemispheres, have distinct cognitive functions. This discovery was made through testing on split-brain patients, individuals whose corpus callosum (the bridge connecting the two hemispheres) had been severed.

Through this research, Sperry showed that one hemisphere dominates in specific cognitive activities, such as logic, language, and reasoning. The left hemisphere dominated in performing these tasks, while the right hemisphere was specialized in spatial orientation and non-verbal cues.

What was Roger Sperry’s hypothesis?

Sperry’s hypothesis was that each hemisphere of the brain had specialized functions and that the corpus callosum’s severing would result in the left and right cortical hemispheres’ isolation. According to Sperry, this isolation would produce distinct cognitive processing in each hemisphere.

His hypothesis was empirically supported through spilt-brain experiments carried out on patients. The results demonstrated that each hemisphere was capable of independent action, demonstrating that the left hemisphere processed cognitive information in a logical and sequential manner, while the right hemisphere processed information in a more intuitive and creative way.

When did Roger Sperry discover left and right hemisphere?

Roger Sperry’s discovery of the left and right hemispheres of the brain was made in the early 1960s. He was awarded the Nobel Prize in Physiology or Medicine in 1981, along with David Hunter Hubel and Torsten Nils Wiesel, for their discoveries concerning how the brain processes information.

What did Roger Sperry experiment?

Roger Sperry’s most famous experiment was known as the Split-Brain experiment, conducted on patients undergoing surgery for severe epilepsy. This surgical procedure required severing the corpus callosum, a bundle of nerves that connects the two hemispheres of the brain.

After the physically separate hemispheres of the brain were isolated, Sperry found that they could no longer communicate with one another. The split-brain patients experienced a disconnection in their behavior and cognitive functions. Things that they would have once been able to do effortlessly became challenging, even to communicating some simple requests.

When did Roger Sperry die?

Roger Sperry died on April 17, 1994, in Pasadena, California, at the age of 80.

Roger Sperry was one of the greatest psychologists of the 20th century. His research on split-brain patients and the function of each hemisphere of the brain has made a profound impact on our understanding of human cognition and behavior.

Sperry’s contribution to neuropsychology and brain research has led to significant breakthroughs in the field and has paved the way for further studies on brain plasticity. His legacy continues to inspire new research, with many scientists building upon his discoveries and contributions.

As proved in the numerous awards he received, including the Nobel Prize, Sperry was a trailblazer who left an indelible mark on the field of neuroscience. His work and dedication to furthering humanity’s knowledge and understanding of the brain will continue to be celebrated and remembered.

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Biopsychology: Evaluating Split-Brain Research

Last updated 10 Apr 2017

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Here are some key evaluation points on split-brain research.

It is assumed that the main advantage of brain lateralisation is that it increases neural processing capacity (the ability to perform multiple tasks simultaneously). Rogers et al. (2004) found that in a domestic chicken, brain lateralisation is associated with an enhanced ability to perform two tasks simultaneously (finding food and being vigilant for predators). Using only one hemisphere to engage in a task leaves the other hemisphere free to engage in other functions. This provides evidence for the advantages of brain lateralisation and demonstrates how it can enhance brain efficiency in cognitive tasks.

However, because this research was carried out on animals, it is impossible to conclude the same of humans. Unfortunately, much of the research into lateralisation is flawed because the split-brain procedure is rarely carried out now, meaning patients are difficult to come by. Such studies often include very few participants, and often the research takes an idiographic approach. Therefore, any conclusions drawn are representative only of those individuals who had a confounding physical disorder that made the procedure necessary. This is problematic as such results cannot be generalised to the wider population.

Furthermore, research has suggested that lateralisation changes with age.  Szaflarki et al. (2006)  found that language became more lateralised to the left hemisphere with increasing age in children and adolescents, but after the age of 25, lateralisation decreased with each decade of life. This raises questions about lateralisation, such as whether everyone has one hemisphere that is dominant over the other and whether this dominance changes with age.

Finally, it could be argued that language may not be restricted to the left hemisphere.  Turk et al. (2002)  discovered a patient who suffered damage to the left hemisphere but developed the capacity to speak in the right hemisphere, eventually leading to the ability to speak about the information presented to either side of the brain. This suggests that perhaps lateralisation is not fixed and that the brain can adapt following damage to certain areas.

• Biopsychology
• Split Brain Research

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Split Brain Research

Hemispheric lateralisation: This is the ideas that the brain’s two hemispheres are responsible for different functions; that particular functions (such as language) are the responsibility of one hemisphere but not the other- the function is lateralised.

Split-brain research: This involves individuals who have had surgical separation of their brain hemispheres, in order to relieve symptoms of epilepsy. Research can reveal to what extent other brain functions are lateralised. Sperry (1968) used a procedure where an image or word was projected to the patient’s right visual field (which would be processed by the left hemisphere) and another image was projected to the left visual field (processed by the right hemisphere). In a split-brain patient, information cannot be transmitted from one hemisphere to another, so the effects of this can be studied.

Findings from split-brain research: When a picture was shown to the right visual field, the patient could describe it easily. When the image was shown to the left visual field, the patient found it difficult to describe it, or couldn’t see anything there. This is likely due to the lack of language processing ability in the right hemisphere, which processes the left visual field. The left hemisphere was unable to receive the information due to the separation of the hemispheres.

When patients were presented with a word or object in the left visual field, then had to select that object (or a related one) from a bag using their left hand without looking, they were able to do this quite well. This is despite the fact that they couldn’t describe what the object was. They could still seemingly understand what the object was, showing that the right hemisphere is involved in understanding of objects.

When patients were presented with two words simultaneously (for example, ‘door’ ‘frame’), one in each visual field, they wrote the word in the left visual field (‘door’) with their left hand, and said the word in the right visual field (‘frame’). The left hand was much better at drawing images than the right hand, despite the fact that all of the patients tested were right-handed. This shows that the right hemisphere seems to be dominant for drawing skills.

When asked to match a face from a number of other faces, the picture presented to the left visual field (processed by the right hemisphere) was consistently selected, whilst the image presented to the right visual field (left hemisphere) was ignored. This shows that the right hemisphere seems dominant in face recognition. Images presented as composites of two faces led to the left hemisphere dominating when the patient was asked to verbally describe the face (the right-hand ‘half’ of the face was described, and the left-hand half ignored), and the right hemisphere dominated when the patient had to select a matching face from a number of others.

Evaluation:

• Sperry’s research supports the conclusion that the left hemisphere is more responsible for verbal and analytical tasks, whereas the right hemisphere is better at spatial and musical tasks. This has strengthened the understanding of how the brain works.
• Sperry’s procedure was closely controlled. Patients were given eye patches, and images were flashed up for a very brief time (fractions of a second), meaning there was no possibility of looking over and using the other visual field. This strengthens the internal validity of the studies.
• The sample used by Sperry was quite small (only 11 took part in all procedures), and their brains may have been affected by epileptic seizures. Therefore, it is hard to generalise the findings from the studies to the general population.

Scanning Techniques

• Functional magnetic resonance imaging (fMRI): this measures changes in blood flow and oxygenation in the brain whilst the individual is performing a task. Increased blood flow suggests that the area of the brain is working harder, so it can be determined which areas of the brain are involved with particular tasks.
• Electroencephalogram (EEG): measures the electrical activity of neurons in the brain, by recording the electrical impulses that take place during synaptic transmission. The individual wears a skull cap to do this. This can detect particular patterns of activity in the brain, for example what is happening during sleep.
• Event-related potentials (ERPs): activity from an EEG recording is analysed, in order to determine the specific responses relating to a particular task. Event-related potentials are therefore the results of this- types of brainwave which are triggered by particular events.
• Post-mortem examinations: the brain is studied and analysed following death, by looking at particular areas. This is often done for individuals who have had rare disorders or dysfunction in behaviour or cognitions. Their brain will be compared to a ‘control’ brain to see if there are differences in structure.
• fMRI is low-risk, involving no radiation, and produces very detailed images. However, it is expensive, and there is a time-lag- the image taken is 5 seconds behind the initial firing of neurons (therefore, this is poor temporal resolution ). Also, the activity of individual neurons cannot be seen.
• EEGs have been very useful in diagnosing conditions such as epilepsy, and the processes involved in activities such as sleep. Brain activity can be measured almost instantaneously (a single millisecond), unlike in fMRI. However, the information gained is very generalised, so the technique can’t be used to isolate exact neural activity.
• ERPs draw on EEGs to measure more specific activity in the brain. They have excellent temporal resolution and are widely used in identifying specific behaviours and functions. Many different ERPs have been successfully identified. However, there is a lack of standardisation in the methodology used, meaning the findings are in question, and eliminating all extraneous variables in order to isolate an ERP can be difficult.
• Post-mortems have greatly enhanced medical knowledge, for example Broca and Wernicke both made use of them before neuroimaging techniques were possible. However, it is hard to establish a cause-effect link when conducting post-mortem studies. Changes in brain structure may not be related to the disorder the patient had, but due to another issue.

Circadian Rhythms

A biological rhythm is a change in the boy’s processes, in response to environmental changes. These rhythms are influenced by external and internal factors. The internal factors are the body’s internal processes- the ‘body clock’, known as endogenous pacemakers . External factors are changes in the environment, known as exogenous zeitgebers . Rhythms that last around 24 hours are known as circadian rhythms .

Sleep/wake cycle: This is an example of a circadian rhythm. The exogenous zeitgeber in this case is the daylight (or lack of it), which contributes to feelings of drowsiness or being awake. Researcher Michael Siffre studied the effect of a complete lack of daylight on his own sleep/wake cycle, by living in a cave for several months at a time. After two months in one cave, he emerged believing the date to be mid-August, but it was actually mid-September. In each experiment, his body created a natural rhythm of just beyond the usual 24 hours, and he continued to sleep and wake on a regular cycle.

Aschoff and Wever (1976) found that participants who spent 4 weeks in a bunker without natural light showed circadian rhythms of 24-25 hours, except one participant who went up to 29 hours. This suggests the natural circadian rhythm is slightly shortened by the effects of daylight. Folkard et al (1985) found that when participants were deprived of sunlight for 3 weeks, and the length of day was manipulated by the researchers to 22 hours rather than 24 (by covertly adjusting the time on the clocks), only one participant easily adjusted to the shortened day. This suggests the strength of the body’s sleep/wake cycle, as it resisted environmental changes.

• Research into circadian rhythms has useful practical applications, for example how to manage the shift patterns of night workers so that they are more productive and make fewer mistakes. This increases the usefulness of the studies.
• Research has also shown when the effects of drugs on the body are at their most and least effective. Circadian rhythms seem to have an impact on how drugs affect the body, so guidelines can be developed for patients as to when they should take drugs for maximum impact. This is another useful practical application.
• Research in this area often uses small sample sizes (only 1, in the case of Siffre), so generalisation may be difficult. Also, participants had access to artificial light, which could have acted as a confounding variable- for example, turning off a light to go to sleep may have similar effects as the end of natural light at the end of a day. The internal validity of the research is therefore in question.

Infradian & Ultradian Rhythms

Infradian rhythms.

These take place over a longer time than 24 hours. For example, the menstrual cycle . This takes place over around 28 days, although this varies between women. Hormone levels rise during the cycle, which causes the release of an egg (ovulation), then the release of the hormone progesterone which thickens the womb lining to ready the body for pregnancy. If no pregnancy occurs, the womb lining comes away, resulting in menstruation.

There is evidence that the menstrual cycle can be affected by exogenous factors. McClintock et al (1998) found that collecting pheromones (chemicals released into the air and absorbed by others, affecting their behaviour) from women with irregular cycles and rubbing them on the lips of other women caused the ‘receiver’ of the pheromones to experience changes in their cycle, bringing them closer to the pheromone ‘donor’.

Seasonal affective disorder (SAD) is another infradian rhythm, characterised by changes to mood. Sufferers feel a lowered mood, and lowered activity levels, during the winter months when daylight is shorter. As their mood changes in a predictable way through the year, this is an example of a circannual rhythm. This is thought to be caused by the hormone melatonin , which has an effect on serotonin, a neurotransmitter linked with depression. More melatonin is released during the winter, as it is released when there is a lack of daylight.

Ultradian Rhythms

These take place more than once within a 24-hour period. An example is the different stages of sleep, of which 5 distinct stages have been identified through research involving an EEG.

• Stages 1 and 2: a light sleep, where a person can be easily woken. Brain wave patterns start to slow down, becoming more ‘rhythmic’ (alpha waves) at this time.
• Stages 3 and 4: delta waves take over, which are even slower than alpha waves. This is a deep sleep; from which it is hard to wave the person- sometimes known as ’slow wave sleep’.
• Stage 5: the pons (part of the brain) paralyses the body to stop the person from ‘acting out’ their dreams. The eyelids move in a fast, jerky fashion at this point, which is correlated with dreaming. Therefore, this stage is known as REM sleep (rapid eye movement).
• Research into the menstrual cycle is likely to be affected by many variables, such as diet, stress, amount of exercise, and so on. This means that the findings of studies such as McClintock may not be valid. Other studies have found no evidence of menstrual synchrony (women’s cycles moving closer to each other’s).
• Dement and Kleitman (1957) found that REM activity was strongly correlated with dreaming. Participants woken during REM sleep were able to describe dreams in vivid detail. This supports the effect of biological rhythms on the body and brain.
• An effective treatment for SAD has been developed as a result of research in this area. Sufferers are given a light box to simulate the effects of sunlight in the dark mornings and evenings of winter, which has led to a relief in symptoms for around 60% of sufferers. This increases the practical usefulness of research into infradian rhythms.

Endogenous Pacemakers

The suprachiasmatic nucleus (SCN): This is located in the brain’s hypothalamus in both hemispheres, and is influential in the maintenance of circadian rhythms. The SCN receives information about light from a structure called the optic chiasm, which sends messages from the eye to the visual area of the cerebral cortex. This can continue even if the person’s eyes are closed, allowing the body to adjust to changing daylight patterns.

Animal studies involving the SCN have shown that if the SCN connections are destroyed, the animals no longer have a sleep/wake cycle- this was observed in chipmunks by DeCoursey et al (2000). In addition, Ralph et al (1990) found that hamsters who received SCN cells through transplant from other hamsters bred to have a 20-hour sleep/wake cycle themselves defaulted to a 20-hour sleep/wake cycle.

The pineal gland and melatonin: The SCN passes information about daylight to the pineal gland, which is located behind the hypothalamus. This gland increases melatonin production, which induces sleep and is inhibited when a person is awake. As previously seen, melatonin is a possible cause of seasonal affective disorder.

Exogenous Zeitgebers

Environmental factors have an influence on biological rhythms through a process known as ‘entrainment’. These factors work with the body’s internal processes to affect rhythms such as the sleep/wake cycle.

Light: This resets the SCN, so has a key effect on the sleep/wake cycle. Hormone production and other processes are also influenced by light. Campbell and Murphy (1998) found that skin can detect light- when participants had light shone on to the back of their knees, this affected the duration of their sleep/wake cycles- even if it was dark outside. This suggests that light is perceived not just by the eyes, and has an effect on the body.

Social cues: Infants have no set sleep/wake cycle until about 6 weeks of age, and this process generally continues until around 16 weeks, when babies are entrained. This could be due to the schedules imposed on them by parents. Similarly, the effects of jet lag can be reduced by quickly adapting to local times for sleeping and eating (not going to bed when you feel tired, for example). This suggests that the body does respond to cues in the environment.

• Damiloa et al (2000) found that other organs in the body have their own circadian rhythms in cells known as ‘peripheral oscillators’. Changing feeding patterns in mice led to changes in the rhythms of the mice’s livers, for example. This supports that there are many influences on circadian rhythms, aside from the SCN.
• Animals used in this research are often exposed to great harm, for example in the DeCoursey study many of the chipmunks were killed by predators after their sleep/wake cycle was destroyed. This raises the question of whether the research is ethically justifiable.
• Evidence suggests that exogenous zeitgebers may not actually have much of an effect on biological rhythms. Miles et al (1977) reported that a man who was blind since birth and had a sleep/wake cycle of 24.9 hours could not have his cycle adjusted by any external factors such as social cues. Instead, he had to take sedatives at night and stimulants in the morning so that he could live in the ’24-hour world’. This weakens the influence of exogenous zeitgebers on biological rhythms.