|Year : 2014 | Volume
| Issue : 2 | Page : 27-33
Split brain syndrome: One brain but two conscious minds?
Divya Agrawal1, Biswa Bhusan Mohanty1, Sanjay Kumar2, Prafulla Kumar Chinara1
1 Department of Anatomy, Institute of Medical Sciences and Sum Hospital, Siksha O Anusandhan University, Bhubaneswar, Odisha, India
2 Department of Pharmacology, Institute of Medical Sciences and Sum Hospital, Siksha O Anusandhan University, Bhubaneswar, Odisha, India
|Date of Web Publication||4-Feb-2015|
Department of Anatomy, Institute of Medical Sciences and Sum Hospital, B. O. Ghatikia, Bhubaneswar - 751 003, Odisha
Source of Support: None, Conflict of Interest: None
The corpus callosum is the largest bundle of commissural fibers connecting the two cerebral hemispheres. The human brain actually functions as two individual brains capable of highly organized mental functions. The dominant hemisphere deals with speech, written language, mathematics, and grammar, whereas the non-dominant hemisphere deals with music, pictorial representation, spatial and temporal synthesis. When this connection is divided surgically, the patient shows what are known as disconnection syndromes which can be classified as acute and chronic. Based on the review of literature, we have tried to deal with the symptom complex of patients who have undergone commissurotomy as treatment for multifocal epilepsy. A selective literature search using the internet and e-library facilities was performed on the topic. The work of great neuropsychologists like Roger Sperry and M. S. Gazzaniga was studied and the differences in functions of the dominant and non-dominant hemispheres were compared. Based on this literature search, it was found that the corpus callosum helps in interhemispheric transfer of information and it is the co-ordination between the two hemispheres which allows us to perform activities smoothly and perfectly. If this connection is severed, actions performed by one hand cannot be understood by another. It was seen that speech, calculation, reasoning, personality, and intelligence are almost completely preserved after commissurotomy. However, cognitive impairment, abstract reasoning, short-term memory, and attention deficits have been reported. All the studies carried out in this field have suggested that the separation of the hemispheres creates two different spheres of consciousness within a single cranium. By using the split brain model, it can be concluded that a normal person's mind is the result of interaction between two separate states of consciousness.
Keywords: Brain, cerebral hemisphere, corpus callosum, disconnection
|How to cite this article:|
Agrawal D, Mohanty BB, Kumar S, Chinara PK. Split brain syndrome: One brain but two conscious minds?. J Health Res Rev 2014;1:27-33
|How to cite this URL:|
Agrawal D, Mohanty BB, Kumar S, Chinara PK. Split brain syndrome: One brain but two conscious minds?. J Health Res Rev [serial online] 2014 [cited 2021 Apr 15];1:27-33. Available from: https://www.jhrr.org/text.asp?2014/1/2/27/150793
| Introduction|| |
The complexity of the brain is a result of its intricate connectivity which is seen by the fact that there is huge rise in white matter during the course of evolution.  Corpus callosum is the largest white bundle of connecting fibers in the floor of median longitudinal cerebral fissure, which consists of about 190 million axons responsible for interhemispheric transfer of information.  The corpus callosum is divided from before backward into four parts which are the rostrum, genu, trunk, and splenium as shown in [Figure 1].
|Figure 1: Gray's anatomy for students showing the lateral view of corpus callosum|
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The genu is the curved anterior extremity which lies 4 cm behind the frontal lobe. The fibers of genu sweep forward on either side into the frontal lobes forming forceps minor. Rostrum is the part which extends downward and backward forming the floor of anterior horn of lateral ventricle and connects the orbital surfaces of the two frontal lobes. The main part of corpus callosum [Figure 2] which forms the roof of central part of lateral ventricle is called the trunk. The splenium is the massive posterior extremity of corpus callosum. These fibers connect the posterior part of parietal lobes, temporal, and occipital lobes which project above calcarine sulcus forming a foot-like structure called forceps major [Figure 2]. This large bundle of commissural fibers helps in transfer of information which takes place at great speed and without our conscious knowledge. Since the two halves of the cerebrum process information differently, when the two co-ordinate, we are able to perform smooth actions as per our personalities. After a lot of debate, neuroscientists have come to the conclusion that the connections are primarily excitatory across the hemispheres. , It is believed that the left-side hemisphere in a right-handed person, i.e. the dominant hemisphere, deals with language and logic, whereas the right hemisphere is in charge of art, music, and imagination. [Table 1] and [Figure 3] It is accepted that right side of the brain controls the left side.
|Figure 2: Gray's anatomy 39th edition showing the superior aspect of corpus callosum|
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|Table 1: Comparisons in the functions of left and right cerebral hemispheres|
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The theory of brain dominance was given by Ned Hermann who is called as "the Father of brain dominance technology". It was based on the work of Sperry. He was inspired by this model to divide the brain into four quadrants [Table 2] according to their peculiarities.
|Table 2: Functional subdivision of brain into four quadrants and their specific activities|
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| WHAT IS SPLIT BRAIN?|| |
In 1930s, the first split brain procedure was carried out in order to prevent the spread of intractable multifocal epileptic seizures from one hemisphere to another.  Epilepsy is a kind of storm in the brain, as a result of excessive signaling of nerve cells. By doing the split brain procedure, this electrical communication would be cut off greatly, thereby reducing the number and severity of the epileptic seizures. The original procedure was known as commissurotomy which involved severing a number of interhemispheric fibers like anterior commissure, hippocampal commissure, and massa intermedia of the thalamus, along with corpus callosum. Later on, only callosotomy, i.e. sectioning of corpus callosum alone, was introduced [Table 3]. There are no significant differences between the two patient groups, and so both commissurotomy and callosotomy patients are called split brain patients.  Multiple seizure disorders are one of the commonest indications for performing commissurotomy. ,
The split brain procedure is seen to have very little effect on day-to-day life.  These patients can do daily chores, drive a car, and do all household jobs.
Early scientists were baffled by the "social ordinariness" as they failed to detect cognitive impairment after the operation.  But after further research, a complex array of deficits was revealed.
The term "partial split" refers to a procedure in which there is sectioning of only anterior two-thirds of the corpus callosum as some seizure disorders respond well to it. Also, partial sections can be used for various conditions like genu section for aneurysm clipping of anterior communicating artery, sectioning of trunk of corpus callosum for access to 3 rd ventricle, and to approach the pineal region by sectioning of splenium. Infectious lesions of the corpus callosum, multiple sclerosis, gliomas in genu or splenium, anterior communicating artery aneurysm, etc., can result in partial or complete dissection of corpus callosum. 
Sectioning of corpus is widely accepted, relatively safe, clearly needed, broadly practiced, and favorable for patients demonstrating life-threatening psychomotor behavior so that they can lead a normal life.
Another condition associated with complete absence or agenesis of corpus callosum is referred to as corpus callosum agenesis. It is the most common brain malformation observed in humans.  Its estimated prevalence is 3-7/1000 births, while in children with developmental disabilities, the prevalence is 2-3/100. ,,
| Disconnection syndromes|| |
A healthy brain looks like two markedly different machines cabled together and exchanging a huge load of data. But this drastic procedure called callosotomy disconnects the two cerebral hemispheres, i.e. neocortex which is the home of conscious thought, language, and movement.
Besides callosotomy, the other causes of disconnection syndromes are:
- Multiple sclerosis
- As mentioned above, agenesis of corpus callosum, rarely because of early plasticity and reorganization of subcallosal channels
- Lesions in corpus callosum in Marchiafava-Bignami disease, a rare alcoholism-related condition
- Hemorrhagic dissection of the callosum due to anterior cerebral artery aneurysm rupture from toxic or infectious lesions of the callosum. ,
When the corpus callosum is destroyed due to any of the reasons mentioned above, the connection between the two hemispheres is interrupted and information like words, objects, pictures, etc., presented to one hemisphere goes unnoticed to another. 
Dissection syndromes are classified as acute and chronic depending on the time that has elapsed after surgery.
Acute disconnection dyndrome
Differences between the two hemispheres have been explored in a number of patients. The left hemisphere leads the way for speech and language computation and the right hemisphere for facial recognition and visual-spatial processing. Splitting the corpus callosum shows that though the two hemispheres are very competent, they provide us with two different pictures of the world. 
Roger Sperry, a neurobiologist and neuropsychologist at California Institute of Technology, and Gazzaniga, a graduate student, conducted a number of animal experiments to explore the lateralized nature of human brain. Researchers in 1940s had concluded that separation of the two hemispheres did not noticeably affect thought and behavior. But Sperry and colleagues revealed very much altered brain function in animals that had undergone callosal section.
When corpus callosum of the right-handed, left hemisphere dominant patient is severed, there is mutism, imperviousness, mild akinesia, and competitive movement between two hands. It is seen that the patient shows repetitive grasping or groping with left hand, left arm hypotonia, and left side apraxia to verbal commands. In early postoperative period, there are situations in which the patients complain that their left hand behaves like a foreigner and they are surprised at purposeful left-hand actions. 
chronic disconnection syndrome
Patients who have undergone complete callosotomy and have recovered from acute postoperative stage manifest a variety of symptoms called chronic dissection syndrome. 
I. Social ordinariness
The patients are indistinguishable from normal people in ordinary social situations except having some memory problems. Special testing only can expose the problem.
II. Loss of interhemispheric transfer of information
This is shown by the fact that they are unable to retrieve with one hand an object which has been palpated by the other hand.
III Hemispheric specialization effects
This can be demonstrated by the fact that right handers will not be able to name or describe an object in the left hand when it is being properly manipulated.
IV Compensatory behavior
So, to deal with all these problems, gradually the split brain patient develops a number of strategies like the patient speaks loudly the name of the object being palpated in the right hand, and as the right hemisphere can recognize many words, the object can be retrieved by the left hand.
Acute and chronic disconnection syndromes and their differences are as summarized in the [Table 4] below.
|Table 4: A comparison of symptoms in acute and chronic disconnection syndrome|
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Though as such, the patients appear normal, long-term interaction reveals memory deficits, low memory intelligence quotients, difficulty in reading stories, and difficulty in acquiring new information.  Also, they show some pragmatic deficits in their conversations, extreme politeness, tendency for confabulation, alexithymia and anomia. ,
In patients showing social ordinariness, lateralized testing is done in which stimuli restricted to one hemisphere show a dramatic lack of communication between two severed hemispheres each of which has its own learning, memory, and perception. So, a right-handed patient who has undergone commissurotomy with left hemisphere speech cannot name stimuli in the left sensory field. 
When different auditory stimuli that are acoustically similar are presented simultaneously to the two ears of patients, they cannot name the left ear stimuli. , This is probably due to suppression of ipsilateral auditory pathway.
Interhemispheric transfer of information for touch, pressure, and proprioception does not take place. Sperry also showed that hand postures impressed on unseen hand by the examiner cannot be copied by the opposite hand. Also, a brief flash of a hand form to one visual field cannot be mimicked by the opposite hand.  So, it has become clear that visual information does not move between the two sides after commissurotomy. If we project an image to the right visual field, i.e. to the left cerebral hemisphere which is where information from the right field is processed, the patient can describe what he saw. But when the same image was projected in the left field, the patients said they could see nothing; yet, if they were asked to point a similar object, they could do so easily, but could not talk about what they saw [Figure 4].
Same was true for touch, sound, and smell. Also, muscles helping in hand and finger movement could be only controlled by the opposite hemisphere, so the right hemisphere controls the left hand and vice versa.
Individuals who have undergone commissurotomy show absence of callosal transfer of sensory information , and also a deficiency in bimanually coordinated motor activity.  Motor skills learned before surgery, such as cooking, biking, guitar playing, swimming, tying shoe laces, etc., remain intact. But tasks requiring bimanual interdependant control, like in etching a sketch, are severely impaired.  Also, it was seen that after complete commissurotomy, the patient was not able to name, but could signal with one hand regarding odors he could smell with right nostril. 
Attention involves structures in cortex and subcortex. It was found that each hemisphere is able to direct spatial attention not only to its own sensory sphere but also to the contralateral hemisphere. This shows that attentional system is common for both hemispheres and can still work after callosotomy using some remaining interhemispheric connections. There are suggestions that attention system in left hemisphere is more object-based and that in the right is more based on location.
The disconnected right hemisphere was superior to the left hemisphere in memory of tactile shapes either when the responses were signaled by touch or by drawing,  and the left hemisphere was superior in tactile or visual memory both for [Table 2] objects and shapes.  There is also creation of false memories which mainly originate in the left hemisphere. Researchers have been working to find out where and how fake memories develop. Some suggest that they develop early in the cycle and erroneous accounts are actually encoded at the time of event. Some believe that false memories reflect an error in reconstructing past experience, i.e. they fit untrue events while recollecting original experience. 
The upper limit of linguistic abilities in each hemisphere varies from subject to subject. The possibility that the right hemisphere has not only some language but also some speech capabilities cannot be ruled out. In many tests "cross-cueing" between two hemispheres [Table 5] may be responsible. 
|Table 5: Typical common findings reported in split brain patients are illustrated in the following table|
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In front of a patient, either a red or green light was flashed to the right hemisphere; the patient could guess the color at chance level, as might be expected if speech mechanism is solely represented in the left hemisphere. If the patient was allowed a second guess, the score improved. Soon the strategy of the patient was revealed as when red light was flashed and the patient answered red, he would stick to the answer. But if the flash light was red and he guessed green, he would frown, shake his head, and say that he meant red. What was happening was that actually the right hemisphere was seeing red, but the left hemisphere guessed green. Since the right hemisphere knew the answer was wrong, the patient shook his head and frowned which gave a clue to the left hemisphere to correct the answer. 
It was found that in certain other mental processes, the right hemisphere is in par with left, particularly in demonstrating an emotional reaction. In one of his experiments, Michael S. Gazzaniga presented a series of ordinary objects and then suddenly flashed the picture of a nude woman. When the picture was flashed to the left hemisphere of a lady patient, she laughed and identified the picture as a nude person. But when it was presented to the right hemisphere, she said she saw nothing, but then she chuckled and a smile spread over her face. This shows that the right hemisphere could not describe what it had seen, but it elicited an emotional response [Table 6].
|Table 6: Effects of split brain procedure on memory, control, and attention are as listed below|
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Hence, the following can be stated from the above data regarding split brain procedure:
- The transmission of abnormal electrical signals between the hemispheres of the brain is considerably reduced or inhibited
- The number and severity of seizures in epileptic sufferer patients is considerably reduced
- The left hemisphere handles the bulk of language processing, but the right hemisphere does have some limited language capacity
- The Sperry research also suggested that the right hemisphere handles the bulk of visual and spatial information
- The split brain procedure is considered to have a minimal impact on everyday behaviors and interactions. 
Taking into account all the evidences, it is seen that separation of hemispheres creates two independent spheres of consciousness within a single cranium. Thus, the two hemispheres are similar, but do not have identical concepts of self, past and future, family, social culture, and history. After some testing with patients, doctors refer the two hemispheres as two distinct people, e.g. they say left hemisphere was upset with the responses of right hemisphere today.  Various workers have demonstrated the effects of split brain surgery on attention, memory and behaviour. ,,,,,,,
So, using a split brain model, a normal individual's mind can be concluded as a result of interaction between two distinct states of consciousness. Scientists often wonder as to why a normal person with intact brain experiences consciousness as unified rather than dual. This is partly explained by the left hemisphere dominance, passive right hemisphere, and a unified system of motor control. Lastly, the famous line quoted by Michael Gazzaniga is: "Don't leave home without your left hemisphere!"
So, the next time you think you are "of two minds" about something, think again!
| Acknowledgments|| |
The article was drafted under the guidance of the author's professor and guide, Chinara P. K., who has many PUBMED and SCOPUS indexed papers to his credit. Also, Dr. Bijaya Bhusan Nanda, former biostatistician, SCB Medical College, Utkal University, Cuttack, India who checked the final manuscript several times is acknowledged. The author would like to thank Dr. R. P. Mohanty, Vice Chancellor, SOA University for instilling a scientific temperament in all of them to work harder and do research. The author would also like to acknowledge the help of Mr. Ramesh Kumar Behera, a trained artist in anatomy department, SOA University, India, who helped with diagrams.
| References|| |
Schoenemann PT, Sheehan MJ, Glotzer LD. Prefrontal white matter volume is disproportionately larger in humans then in other primates. Nat Neurosci 2005;8:242-52.
Tomasch J. Size, distribution and number of fibres in the human corpus callosum. Anat Rec 1954;119:119-35.
Bloom JS, Hynd GW. The role of the corpus callosum in interhemispheric transfer of information: Excitation or inhibition? Neuropsychol Rev 2005;15:59-71.
Morris RJ. Left brain, right brain, whole brain? An examination into the theory of brain lateralization, learning styles and the implications for education, PGCE Thesis; Cornwall College St Austell. 2006. Available from: http://singsurf.org/brain/rightbrain.html
[Last retrieved on 2014 Oct 23].
Reeves AG, Roberts DW, editors. Epilepsy and the Corpus Callosum 2. New York: Plenum Press; 1995. p. 183-90.
Bayne T. The unity of consciousness and the split brain syndrome. J Philos 2008;105:277‐300.
Schwartz TH. Corpus callosotomy: Indications and technique (Stereotactic and functional neurosurgery). What-when-how in depth tutorials and information. Available from: http://what-when-how.com/stereotactic-
and-functional-neurosurgery/corpus-callosotomy- indications-and-technique-stereotactic- and-functional- neurosurgery/[Last retrieved on 2014 Oct 23].
Asadi-Pooya AA, Sharan A, Nei M, Sperling MR. Corpus callosotomy. Epilepsy Behav 2008;13:271-8.
Zaidel DW. A view of the world from split-brain perspective. In: Critchley EM, editor. The Neurological Boundaries of Reality. London: Farrand Press; 1994. p. 161-74.
Akelaitis AJ. A study on gnosis, praxis and language following section of the corpus callosum and anterior commissure. J Neurosurg 1944;1:94-102.
Zaidel E, Zaidel DW, Bogen JE. Testing the commissurotomy patient. In: Boulton AA, Baker GB, Hiscock M, editors. Neuromethods. Methods in Human Neuropsychology. Vol. 15. Clifton, NJ: Humana Press; 1990. p. 147-201.
Dobyns WB. Absence makes the search grow longer. Am J Hum Genet 1996;58: 7-16.
Grogono JL. Children with agenesis of the corpus callosum. Dev Med Child Neurol 1968;10:613-6.
Jeret JS, Serur D, Wisniewski K, Fisch C. Frequency of agenesis of the corpus callosum in the developmentally disabled population as determined by computerized tomography. Pediatr Neurosci 1985-1986;12:101-3.
Glass HC, Shaw GM, Ma C, Sherr EH. Agenesis of the corpus callosum in Callifornia 1983-2003: A population-based study. Am J Med Genet A 2008;146A: 2495-500.
Wolman D. A Tale of Two Halves. Nature 2012;483:260-3.
Bogen JE. Partial hemispheric independence with the neocommissures intact. In: Trevarthen C, editor. Brain Circuits and Functions of the Mind. Cambridge: Cambridge University Press; 1990. p. 215-30.
Zaidel DW. Memory and spatial cognition following commissurotomy. In: Boiler F, Grafman J, editors. Handbook of Neuropsychology. Vol. 4. Amsterdam: Elsevier; 1990. p. 151-66.
Devinsky O, Eposito MD. Neurology of cognitive and behavioural disorders. Contemporary neurology series. Oxford University Press; 2004. p. 194-6.
Bogen JE, Vogel PJ. Neurological status in the long term following cerebral commisurotomy. In: Michael F, Schott B, editors. Les Syndromes de Disconnexion Calleuse Chez, L′Homme. Lyon: Hopital Neurologique, 1975.
Sidtis JJ. Dichotic listening after commissurotomy. In: Hugdahl K, editor. Handbook of Dichotic Listening. New York: Wiley; 1988. p. 161-84.
Sperry RW. Mental unity following surgical disconnection of the cerebral hemispheres. The Harvey Lectures. Series 62. New York: Academic Press; 1968. p. 293-323.
Bogen JE. The callosal syndromes. In: Heilman KM, Valenstein E, editors. Clinical Neuropsychology. New York: Oxford University Press; 1993. p. 337-407.
Bogen JE. Split-brain syndromes. In: Frederiks JA, editor. Handbook of Clinical Neurology. Amsterdam: Elsevier; 1985. p. 99-106.
Zaidel D, Sperry RW. Some long-term motor effects of cerebral commissurotomy in man. Neuropsychologia 1977;15:193-204.
Gordon HW, Sperry RW. Lateralization of olfactory perception in the surgically separated hemispheres of man. Neuropsychologia 1969;7:111-20.
Levy J. Psychobiological implications of bilateral asymmetry. In: Dimond AJ, Beaumont JG, editor. Hemisphere function in the human brain. New York: John Wiley and Sons; 1974.
Gazzaniga MS. The split brain revisited. Sci Am 1998;279:50-5.
Gazzaniga MS. The split brain in man. Sci Am 1967;217:4-29.
Janet M, Funnell M, Gazzanuga M. Right-hemisphere memory superiority: Studies of split-brain Patient. Psychol Sci 1995;6:157-64.
Sergent J, Corballis MC. Human perception and performance. J Exp Psychol 1989;15:701-10.
Farah JM. The laterality of mental image generation: A test with normal subjects. Neuropsychologia 1986;24: 541-51.
Levy J, Trevarthen C. Metacontrol of hemispheric function in human split-brain patients. In: Reeves AG, Roberts DW, editors. Epilepsy and the Corpus Callosum 2. New York: Plenum; 1995;2:299-312.
Nebes RD. The commissurotomized brain. In: Boiler F, Grafman J, editors. Handbook of Neuropsychology. Vol. 4. Amsterdam: Elsevier; 1990. p. 3-168.
Tramo MJ, Baynes K, Fendrich R, Mangun GR, Phelps EA, Reuter-Lorenz PA, et al
. Hemispheric specialization and interhemispheric integration: Insights from experiments with commissurotomy patients. In: Reeves AG, Roberts DW editors. Epilepsy and the Corpus Callosum 2. New York: Plenum; 1995. p. 263-95.
Zaidel E. Interhemispheric transfer in the split brain: Long term status following complete cerebral commissurotomy. In: Davidson RH, Hugdahl K, editors. Human Laterality. Cambridge, MA: MIT Press; 1994. p. 491-532.
[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]