The Curious Case of the People With Split Brains

In late 1961, Drs. Philip Vogel and Joseph Bogen, neurosurgeons at the California College of Medicine in Los Angeles, were preparing to carry out a radical new procedure. The patients under their care suffered from severe epilepsy, which despite their doctors’ best efforts had resisted all attempts at conventional treatment.  One such patient, a 48-year-old former paratrooper identified in medical records as W.J, had suffered a head injury during a combat jump in WWII and soon began experiencing frequent blackouts and convulsions, with one particularly severe episode in 1953 lasting three full days. Vogel and Bogen suspected that these seizures were amplified by rogue neural signals spreading from one side of the brain to the other, and hoped that their severity might be reduced by interrupting communication between the two. This procedure, known as a corpus callosotomy, involved severing the corpus callosum, the bundle of white matter that bridges the two hemispheres and allows signals to cross from one to the other.

Vogel and Bogen had every reason to believe the procedure would work, as experimental corpus callosotomies performed on cats and monkeys had produced few observable side effects. Psychologist Karl Lashley had even speculated that the corpus callosum served no greater purpose than to “keep the hemispheres from sagging.” And indeed, upon waking from their surgeries the epileptic patients seemed entirely normal, with one even quipping that he had a “splitting headache.” But as they recovered, it soon became apparent that something wasn’t quite right. The patients began to favour the right side of their bodies in everyday activities, and seemed oblivious to any stimulation coming from the left side. For example, if they bumped their left arm they would not notice, and if an object was placed in their left hand they would deny its existence. Intrigued by this strange behaviour, in 1962 psychologists Roger Sperry and his graduate student Michael Gazzaniga of the California Institute of Technology began a series of groundbreaking experiments to find out just what was going on inside the split-brain patients’ heads. What they discovered would change our understanding of the human brain forever.

It has long been known that the brain’s functions are not evenly distributed, with each hemisphere specializing in different tasks. For example, in 1861 French physician Paul Broca discovered that damage to a specific region of the left frontal lobe – a region now known as Broca’s Area – resulted in various forms of aphasia – the inability to speak or understand words. This and other observations lead to the left hemisphere being recognized as the primary language centre of the brain. But until the experiments of Sperry and Gazzaniga, neurologists did not truly understand the full extent of this specialization. The split-brain patients provided a golden opportunity to study hemispheric specialization, for the surgery had essentially left them with two, nearly-independent brains. Contrary to Karl Lashley’s dismissive appraisal, the corpus callosum is in fact a highly sophisticated and essential part of the brain, containing some 200 million neural fibres capable of transmitting one billion bits of information per second. This is vital to normal functioning of the brain, for due to a quirk of vertebrate evolution our nervous systems are contralateral, meaning that each hemisphere receives information largely from the opposite side of the body. For example, the optic nerves, which convey visual information from our eyes to our occipital lobes, cross over at a junction called the optic chiasm, meaning that information from the right eye is transmitted to the left hemisphere and vice-versa. Ordinarily this counter-intuitive arrangement works just fine as the information is immediately transmitted to the correct hemisphere via the corpus callosum. But in split-brain patients this channel of communication no longer exists, meaning that information transmitted to a particular hemisphere stays in that hemisphere. And this is where things start to get weird.

Sperry and Gazzaniga probed the patients’ hemispheres individually by stimulating the opposite side of the body – for example, by presenting an image to the right eye to stimulate the left hemisphere. In one early experiment, they flashed a series of lights across the patients’ field of view. When asked to report when they had seen a light, the patients only reported seeing lights flashing on the right. But when asked to point whenever they saw a light, they successfully reported seeing lights on both sides. Next, Sperry and Gazzaniga projected the word HEART such that the letters HE appeared in the patient’s left-hand field of vision and ART in their right-hand field of vision. When asked to report what they saw, the patients verbally responded “ART”; but when asked to point to the word they saw using their left hand, they pointed to HE. Similarly, if an object was placed in the patient’s right hand, they were easily able to name it, though when asked to point to an image of the same object using their right hand,  were unable to do so. When the sides were reversed, the patients could easily point to the object, but, much to their confusion, were unable to name it. This and similar experiments indicated that language processing abilities are almost entirely localized in the left hemisphere, while the right hemisphere specializes in visual perception tasks such as recognizing faces and emotions and spotting differences between objects. This verbal-perceptual divide is even baked in from birth, with most infants favouring the left side of their mouths when smiling and the right side when babbling.

But as is often the case in biology, Sperry and Gazzaniga soon discovered that things weren’t quite so clear-cut, and that the right hemisphere was a far more capable communicator than previously believed. For example, one patient, when presented with a picture of his girlfriend in his left eye, was unable to speak her name, but was able to spell it out using Scrabble tiles. Sperry and Gazzaniga also found that while the left hemisphere excels at making straightforward word associations, the right hemisphere is better at recognizing subtler relationships and insinuations. For example, when the left hemisphere was presented with the word foot, it was better at picking out a related term like heel from a list of words. But when the right hemisphere was presented with two additional words, cry and glass, it more easily picked out the connecting word – in this case, cut.

But as strange as these discrepancies are, the experience of living with a split brain can sometimes be even more bizarre, with patients feeling as though they literally have two separate brains – brains which are often at odds with one another. For example, patients have reported doing up their shirt buttons with one hand only to have the other hand spontaneously unbutton them, or placing items in a shopping cart with one hand only for the other to place them back on the shelf. Many patients are even able to copy two different images using each of their hands, though given the right hemisphere’s greater spacial reasoning capabilities, the left hand is generally superior at this task than the right. In rare cases this phenomenon can even take the form of “alien hand syndrome,” in which a patient’s hand appears to have a mind of its own and sometimes attempts to strangle its owner or others. This is also sometimes known as “Dr. Strangelove Syndrome” after Peter Sellers’ character in the 1964 Stanley Kubrick film who exhibits similar symptoms. Unfortunately there is no cure for the condition other than keeping the offending hand occupied with other tasks and restraining it at night to prevent injuries.

In such cases of independent limb movement, the offending limb is almost always the left one. This reflects what is perhaps Sperry and Gazzaniga’s greatest discovery: the executive dominance of the left hemisphere. Their and later experiments revealed that much of the right hemisphere’s reasoning and decision-making processes are entirely unconscious and must be mediated and interpreted by the dominant left hemisphere for us to become aware of them. When the connection between the hemispheres is severed, this mediation and interpretation function is lost, hence why the left-hand limbs – controlled by the right hemisphere – are able to act without their owner’s conscious knowledge. This effect also applies to logical reasoning. For example, when one split-brain patient was shown a picture of a chicken foot in their right-hand visual field and a snowy field in their left-hand visual field and asked to choose the closest association from a list of words, they paired a chicken with the chicken foot and a shovel with the snowy field. However, when asked to rationalize why they had chosen the shovel, the patient responded “to clean out the chicken coop.” This indicated that the image presented to the left hemisphere – the chicken coop – had overridden that presented to the right hemisphere in the patient’s mind.

This extreme divide between the functions, capabilities, and even the “personalities” of the two hemispheres stunned the psychology community, and lead Roger Sperry to conclude in 1974 that:

“…[each hemisphere is] indeed a conscious system in its own right, perceiving, thinking, remembering, reasoning, willing, and emoting, all at a characteristically human level, and … both the left and the right hemisphere may be conscious simultaneously in different, even in mutually conflicting, mental experiences that run along in parallel.”

Sperry and Gazzaniga’s experiments revolutionized our understanding of how the brain organizes different perceptual and conceptual tasks, and for this work Sperry, along with David Hubel and Torsten Wiesel, were awarded the 1981 Nobel Prize for Medicine and Physiology. Sperry continued to study split-brain patients until his death in 1994, while Gazzaniga still pursues this line of research to this day.

If all this talk of hemispheric specialization sounds a bit familiar, it may be due to the popular belief that people are predominantly “left” or “right-brained” depending on their particular cognitive strengths. The reasoning goes that since the left hemisphere specializes in language and logical reasoning and the right hemisphere in visual and spacial reasoning, those with a mathematical or scientific bent are predominantly “left-brained” while those with a more creative, artistic temperament are “right-brained.” But just like the notion that we only use 10% of our brains, the whole “left-brained”/“right-brained” dichotomy is nothing but a load of bunkum. Numerous studies using functional MRI have shown that both sides of the brain are used more or less equally regardless of the cognitive task being performed. And this makes sense, as supposedly “left-brain” fields like mathematics and science are also profoundly creative activities, while supposedly “right-brain” activities like art often require a great deal of analytical precision. So, sorry, but all those online quizzes promising to reveal your cognitive style are about as scientific as a horoscope.

As for the split-brain patients themselves, the vast majority reported a significant reduction in the frequency and severity of their seizures. While this improvement came at the cost of living with a pair of often uncooperative brains, most eventually learned to cope with this strange existence in a variety of fascinating ways. Sperry and Gazzaniga even observed one of these adaptations, which they called “cross-cuing,” in one of their experiments. This experiment involved flashing either a red or green light in the patient’s left-hand visual field and asking them to report which colour they had seen. If the patient answered correctly more than half the time – as would be expected if they were just guessing – this would indicate that the right hemisphere had at least some spoken language ability. Strangely, when the experimenters allowed the patient to make a second guess, their scores improved dramatically. After a while, Sperry and Gazzaniga realized that when the patient’s right hemisphere saw one colour but heard the patient say the other, it unconsciously caused the patient to frown. The patient’s left hemisphere then detected this frown and deduced that it had guessed wrong. In this manner, split-brain patients are able to use subtle physical cues to allow their right and left hemispheres to communicate to a limited extent. Studies of children born without a corpus callosum also indicate that the hemispheres also communicate via other means, meaning that split-brained patients’ brains may not be as disconnected as we once thought. To paraphrase Jeff Goldblum in Jurassic Park: the brain, uh, finds a way.

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Expand for References


Myers, David, Psychology, Worth Publishers, NY, 2004


Experiment Module: What Split Brains Tell Us About Language, McGill University,


Gazzaniga, Michael, The Split Brain in Man, Scientific American 217(2), 1967,


Metcalfe, Janet; Funnell, Margaret & Gazzaniga, Michael, Right-Hemisphere Memory Superiority: Studies of a Split-Brain Patient, University of California Davis, May 1995,


No, You’re Not Left-Brained or Right-Brained, Psychology Today, February 15, 2018,

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