What The Split Brain Reveals About The Nature Of Consciousness

“Conscious experience is at once the most familiar thing in the world and the most mysterious. There is nothing we know about more directly than consciousness, but it is far from clear how to reconcile it with everything else we know.”

– David J. Chalmers

WE ARE ALL intimately familiar with that voice in our heads, nagging as we reach for a second helping of chocolate eggs in the wake of Easter, and judging as we have to settle for a looser belt hole a week later. We are all also accustomed to the different masks we wear in various social situations. I would be ruthlessly mocked could my close friends see the way I behave around my girlfriend, snuggled on the couch on a Sunday night, and my parents would be hard pressed to recognize me. But this apparent discontinuity is a perfectly normal aspect of human existence – psychologists have long known that situation exerts a significant influence on personality – and I have never experienced anything which would seem to indicate that there was more than one self calling the inside of my skull home. As a matter of conscious experience, I am just as much me whilst goofing around with my girlfriend as I am typing this essay and sipping green tea. But am I really? As a result of a fascinating avenue of inquiry involving patients who have had the hemispheres of their brain surgically separated, psychologists, cognitive scientists, and philosophers have begun to argue that there is every reason to believe in the divisibility of consciousness – the existence of a separate centre of consciousness in each of the divided hemispheres – and you and I might not be as different to these patients as we think.

Flashback to 1962, a man known only as W.J. becomes the first patient in history to have the two hemispheres of his brain completely separated, a procedure known as a corpus callostomy (also known as a commissurotomy). A paratrooper in world war two, W.J. suffered a broken leg and a severe blow to the head after his parachute failed to open during a bombing raid over Holland. Knocked unconscious by the fall, he was captured and taken to as a prisoner of war, suffering a second head injury courtesy of a guard’s rifle stock, damaging the parietal lobe of his brain.

Described as an intelligent and promising young man before his injury, W.J. returned from the war to a job as a payroll clerk and began experiencing black-outs – maintaining consciousness but losing all memory of unknown periods of time. In one such episode W.J. came to after having driven fifty miles, with absolutely no recollection of the experience. The blackouts continued for twelve years, during which he began to experience generalized seizures – a form of epilepsy which affects both cerebral hemispheres and results in a loss of consciousness. Initially only occurring once every two to three months, by 1956 W.J. was suffering from two to three seizures per day, and now aged forty two, finally sought treatment. Hospitalization and a heavy regimen of pharmacotherapy proved to be ineffective, and it was finally decided that was decided that a corpus callostomy – a complete dissection of the corpus callosum – was the only suitable course of action.

The corpus callosum is a large bundle of neuronal fibres which, along with four other major fibre tracts, connects the cerebral hemispheres of the brain. It facilitates the transfer of information between the two hemispheres, a process known as interhemispheric communication. When someone suffers an epileptic seizure, the complex asynchronous firing pattern of neural activity disrupted, and abnormally synchronized firing spreads propagates over large areas of the cortex. Circa 1940s a number of corpus callostomy procedures were carried out in patients with severe epilepsy. It was hypothesized that by severing the major neural highway between the hemispheres the electrical storm of abnormal activity would be relegated to a single hemisphere, reducing the severity and frequency of seizures.

These early procedures, although providing little relief from epilepsy, seemed not to produce any significant behavioural or mental side effects. Disappointed scientists were led to believe that the corpus callosum served little more function than to keep the hemispheres from sagging, and the procedure was abandoned. That is, until two Californian neurosurgeons, Philip Vogel and Joseph Bogen, baffled by the idea that dividing a brain had no significant impact on a person’s functioning, revisited these early experiments. After scrutinizing the details of these initial split brain procedures, Vogel and Borgen were convinced that surgery had only failed to control the patients’ epilepsy because in most cases the corpus callosum had not been completely severed.

In light of these findings, W.J. underwent the first complete split brain surgery, which turned out to be a complete success. After a month he was almost fully recovered; the procedure did not seem to affect his I.Q. or personality, his seizures had subsided, and he was able to return to his life, the quality of which was dramatically improved. Yet W.J. could not have known the impact that his case would have on modern neuroscience and philosophy. As of today, ten patients have had the operation, four of whom have been extensively studied over a number of decades. Split brain research went on to stimulate hundreds of studies into the hemispheric specialization of the brain, led to speculations about the possibility of an independent centre of consciousness in each hemisphere, and captured the attention of the public at large. Indeed “no one, psychologist, philosopher, neurologist, or humanist”, as Donald Hebb, father of neuropsychology and neural networks notes, “is entitled to an opinion on the mind-body question if he is unfamiliar with the split-brain procedure”.

ALTHOUGH IN THE first thirty days after having his hemispheres split, W.J. was unable to speak, he soon recovered his use of language to the point of being about to recite the tongue twister “Peter Piper picked a peck of pickled peppers”, and despite what he described as a “splitting headache”, appeared to be otherwise functioning completely normally. This presented somewhat of a conundrum for researchers Roger Sperry and Michael Gazzaniga, whom had shown in a vast array of experiments with rats, cats, and monkeys, that severance of the cerebral commissures profoundly limited the exchange of information from one hemisphere to the other, and was associated with a host of cognitive deficits.

Sperry and Gazzaniga had first examined the split brain in cats, severing both the cerebral commissures and the optic chiasm – a tract of nerve fibres which carries visual information such that the right half of each eye’s visual field is received by the left occipital cortex, and vice versa. In severing the optic chiasm, the visual field of each eye would only reach the visual cortex on the same side – no visual information would cross the hemispheres. By presenting visual information to only one eye at a time, Sperry and Gazzaniga stumbled upon an incredible finding – an animal would respond normally when presented to visual information one eye at a time, but when that information was presented to the other eye, the animal showed no sign of recognition. This finding absolved the corpus callosum its previously assumed role of simply holding the brain together, suggesting rather, that it served to keep each hemisphere informed about the goings on of the other.

Thus, the finding that complete severance of the corpus callosum in humans produced no noticeable change in temperament, personality, or intelligence left Sperry and Gazzaniga baffled. That would change soon enough, however, as Gazzaniga fondly recalls the moment he, as a graduate student under Sperry, discovered that W.J. was unable to verbally describe stimuli presented to his disconnected right hemisphere. Close observation soon revealed a number of abnormalities in the day-to-day functioning of these split brain patients. For example, patients tended to favour the left-brain-controlled right side of the body when interacting with the world. Conversely, the right-brain-controlled left side of the body rarely responded to stimuli, nor did it spontaneously interact with the environment. Even more surprisingly, W.J. flat our denied the existence of an object placed in his left hand out of sight of his left visual field. These findings gave rise to the now classic split brain studies, in which it was discovered that following separation from each other, each hemisphere functioned independently, as if it were a complete brain.

THE TYPICAL SPLIT brain experiment attempted to take advantage of the subject’s deficit in inter-hemispheric communication by presenting stimuli in such a way that they will be only presented to one hemisphere – the human nervous system, like that of the cat, is arranged in such a way that information presented to the right visual field of each eye is projected to the left hemisphere, and vice versa. In one experiment, the subject is asked to fixate his gaze on a central point on a board.  Spots of lights are flashed across the board for a fraction of a second, presented in a way that they are only visible to one half of the subject’s visual field at a time. When lights were presented to the right half of the visual field – and processed in the left hemisphere – the subject was able to see them perfectly. However, when visual information was presented to only the left visual field – the disconnected right hemisphere – the subject denied seeing anything at all.

Given these findings it might seem logical to conclude that the right hemisphere of the split brain patient is in effect blind, but this is only the case when they are asked to verbally report the presence of flashes. When instead asked to point to where lights had been flashed, the brain’s right hemisphere proved to be practically equal in its visual perception to the left, and the subject would easily point to the location of the flashing lights with his left hand – motor control of the left side of the body is controlled in the right hemisphere. The deficit, therefore, was not in the patient’s ability to see stimuli presented to the right hemisphere, rather it was an inability to verbally report what had been seen. These findings make perfect sense given that, in the majority of people, speech functions are localized to the left hemisphere, regions known as Broca’s and Wernike’s area, the best friends of any undergraduate psychology student. Thus the right hemisphere was entirely aware that it had seen something, and able to communicate this through movements of the body, it simply wasn’t able to tell the left hemisphere what had happened. Similar findings were described in the sensory medium of touch, as objects held in the subject’s right hand (from which information is projected to sensory areas in the left hemisphere) are easily identified, whilst objects held in the left hand (projecting to the right hemisphere) could not be verbally identified, but could be identified via other means, such as pointing to a drawing of the object.

Further investigation revealed that when a word, such as ‘hat’, is presented to the right hemisphere, although the subject cannot verbally report what they have seen, the left hand will pick out a hat from a group of concealed objects. This takes place whilst the subject claims they saw nothing at all! But the peculiarities don’t end there. When an object is placed out of sight in the left hand of a split brain patient and they are asked to guess the identity of the object, wrong guesses will elicit facial expressions of annoyance. This occurs because the right hemisphere, although unable to communicate verbally, still hears the incorrect answer provided by the left hemisphere, and is bothered that the left got it wrong. Meanwhile, a correct guess will elicit a smile. Yet for all these idiosyncrasies displayed in the lab, these split brain patients, by and large, continue to live a normal life. The hemispheres manage to cooperate admirably.

Split brain test p.jpg

From these results two principle findings emerged. The first was that the brain possesses a high level of functional hemispheric lateralization; certain tasks, such as language, are specifically lateralized to the left hemisphere. This wasn’t in itself a ground-breaking discovery, since studies for over half a century had shown that damage to the left hemisphere could cause functional deficits in the use of language. The second finding, however, was much more surprising, with far reaching implications in science and philosophy. It was discovered that each hemisphere displays an astonishing level of functional independence – the possession of separate memories, learning processes, and behavioural intentions, unbeknownst to the other hemisphere. Although the segregated right hemisphere could not express itself verbally, it could respond crudely with actions of the left hand, such as pointing to words, drawing, or spelling simple words. Clearly, although preventing communication between the hemispheres was enough to reduce the verbal capacity of the right hemisphere to that of a one year old, it had not been rendered unconscious. Presenting a picture of a nude woman elicited an amused reaction regardless of the hemisphere to which it was presented, although the subject would of course deny having seen anything – clearly mental life remained in the right hemisphere, the left hemisphere was simply unaware of it. When asked why they had laughed spontaneously the subject would confabulate an answer – “I don’t know…nothing, oh-that funny machine”. At this point it seems difficult to say that the split brain patient remains a single subject, and the logical next question becomes ‘how many minds do these people have?’

ALL OF THE evidence provided by Sperry and Gazzagina seems to indicate that the separation of the hemispheres resulted in the presence of two independent spheres of consciousness within the same organism. The split brain phenomenon provided the first scientific demonstration that the self has a real, physical basis, and one that could, in fact, be divided with a knife. As philosopher Thomas Nagle argues, there seems no legitimate reason to regard the ability to verbalize as a necessary condition of consciousness, nor is there any reason to believe that, upon its separation from the verbal left hemisphere, the right hemisphere was rendered unconscious. It may not be able to communicate verbally, but the right hemisphere is still entirely able of responding to complex stimuli and controlling behaviour. If we consider the alternative case of removing one’s left hemisphere, we would see no reason to consider them now an unconscious automaton due to their loss of speech. Each segregated hemisphere in fact appears to be capable of higher order mental functions, and there is evidence to suggest that the divided hemispheres, functioning as if two separate brains, are capable of handling twice as much information as the normal, whole brain.

Nagle argues, therefore, that there exists sufficient evidence of consciousness in the isolated right hemisphere, complete with its own perceptions, beliefs, and actions. The right hemisphere when isolated from the left, in effect became a separate person. Although disconnected from the left hemisphere in an unusual way, the two halves of the brain still share a common body, controlled by a common midbrain and spinal cord. Just because the verbal left hemisphere cannot report having any knowledge of the goings on of the right hemisphere, there seems no reason at all to suspect that it is not like something to be that right hemisphere. As Sam Harris puts it in his book “Waking Up” – “to ask the left hemisphere what it is like to not know what the right hemisphere is thinking is rather like asking a normal subject what it is like to not know what another person is thinking” – he simply does not know.

Famed neuroscientist and author V.S. Ramachandran cites an especially convincing case for the existence of an independent sphere of consciousness in the right hemisphere, complete with its own separate personality. Ramachandran, after training the right hemisphere to communicate via pointing to yes or no answers, asked the left and right hemisphere if they believed in the existence of God, eliciting a different answer from each hemisphere. Whilst posing a somewhat insurmountable problem for the notion of a soul destined for heaven or hell, this subject’s response does prove that our subjectivity – our sense of self – can quite literally be sliced in two. We therefore, it seems, have every reason to believe that a separate, independent, centre of conscious experience exists in the right hemisphere – and this has remarkable implications concerning the nature of consciousness – that consciousness is divisible.

Consider a thought experiment proposed by Harris. Imagine that you undergo a corpus callostomy, which hemisphere of your brain would “you” end up on? Given the fact that one can remain conscious throughout the entire procedure – thus eliminating the possibility that you would cease to exist and be replaced by two, somewhat similar, selves – it is tempting to conclude that you would end up residing in the left hemisphere – you would, after all, retain the use of language and discursive thought. But the problem here, argues Nagle, is in trying to understand consciousness using our own paradigm of psychological unity; “but in thus using ourselves as the touchstone of whether another organism can be said to house an individual subject of experience or not, we are subtly ignoring the possibility that our own unity may be nothing absolute, but merely another case of integration, more or less effective, in the control system of a complex organism”.

THE ILLUSION OF being a single unified self stems from the use of the first person singular pronoun I – we therefore consider our own sense of self an absolute and singular function of the integration our neurology. The evidence, however, points to the fact that if I enjoy unity of mind – everything I have ever remembered, intended, or perceived has been known by a single “subject” of experience – then it depends entirely on a tract of approximately 190 million axons crossing the midline of my brain. Harris argues, however, that given the immense amount of information processing which occurs independently in each hemisphere – there are approximately twenty billion neurons in the human cortex, each of which is connected to thousands of other neurons – the corpus callosum is simply insufficient to integrate all of this information.

It makes sense, therefore, that the healthy, intact human brain is already split to some degree, a claim that philosopher Roland Puccetti argues the silent right brain has known all along – “the non-speaking hemisphere has known the true state of affairs from a very tender age. It has known this because beginning at age two or three it heard speech emanating from the common body that, as language development on the left proceeded, became too complex grammatically and syntactically for it to believe it was generating”. According to Harris, the split brain patient is no different from you and I, other than the fact that their right hemisphere is missing half of its field of view.

“Whatever the final lesson of the split brain is”, notes Harris, “it thoroughly violates our common sense intuitions about the nature of our subjectivity.” Just as we are lacking any evidence to deny the existence of an independent seat of consciousness in the split brain patients right hemisphere, we are lacking any evidence to indicate the absence of such a separate point of view in our own. Absence of evidence, as it goes, is not evidence of absence. “The point of view from which you are consciously reading these words may not be the only conscious point of view to be found in your brain. It is one thing to say that you are unaware of a vast amount of activity in your brain. It is quite another to say that some of this activity is aware of itself and is watching your every move.” Any attempt to understand the nature of consciousness in a system that is functionally mute is bound to fail as long as we continue to correlate consciousness with self-report. “Each of us may live, even now, in a fluid state of split and overlapping subjectivity”, asserts Harris. “Whether or not this seems plausible to you may not be the point. Another part of your brain may see the matter differently.”


  • Gazzaniga, M. S. (1988). Brain modularity: Towards a philosophy of conscious experience.
  • Gazzaniga, M. S. (1967). The split brain in man. Scientific American, 217(2), 24-29.
  • Gazzaniga, M. S. (1998). The split brain revisited. Scientific American, 279(1), 50-55.
  • Harris, S. (2015). Waking up: A guide to spirituality without religion. Random House.
  • Nagel, T. (1971). Brain bisection and the unity of consciousness. Synthese, 22(3), 396-413.
  • Wolman, D. (2012) The Split Brain: A Tale of Two Halves. Nature.


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I am a science graduate, wannabe freelance writer and newly converted blogger. A nomad at heart, I travel as much as I can whilst treating my mind and body like some kind of twisted laboratory, often testing the mental boundaries of sanity and mental health. I write at theneocortex.wordpress.com, an exploration into the human condition and well-being through the lens of neuroscience, psychology and practical philosophy. I consider myself a humanist, albeit a highly flawed one. Peace Out.

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