Table of Contents

Table of Contents

The Split-Brain Experiment

Patient WJ

In 1962, a patient was part of an experiment that had never been done before. This was Patient WJ, a 48-year-old former World War II paratrooper who had suffered a head injury, and was frequently experiencing convulsions and blackouts. One of his worst episodes was in 1953, and lasted 3 days.


An epileptic seizure is a transient occurrence of signs and/or symptoms due to abnormal excessive or synchronous neuronal activity in the brain. Epilepsy is a disorder of the brain characterized by an enduring predisposition to generate epileptic seizures, and by the neurobiologic, cognitive, psychological, and social consequences of this condition. The definition of epilepsy requires the occurrence of at least one epileptic seizure.

Seizures like this occur because of abnormally high activity in the brain.

A seizure occurs when many neurons in the brain fire simultaneously, often recruiting other cells into this synchrony of firing. In normal brain wave functioning, neurons fire separately, triggering the adjacent neuron to fire. [...] When the synchrony spreads to both sides of the brain, a generalized seizure occurs.

Cutting ties

The neurosurgeons on the case, Drs Vogel and Bogen, and neuropsychologists, Sperry and Gazzaniga, had an idea: what if the connection between the 2 hemispheres of the brain, the corpus callosum, was cut? This would significantly reduce the spread of the seizure, preventing the harm that seizures could cause. Although the surgery itself seemed fairly safe, the effects of the procedure, called corpus callosotomy, were still unknown and could have detrimental effects on the patient.

Previous attempts at corpus callosotomy to treat epilepsy in the 1940s by Dr William P. van Wagenen had ended in failure. However, the scientists had reason to believe that their attempt would be successful.

Researchers who studied the first split-brain patients in the 1940s had concluded that the separation didn't noticeably affect thought or behaviour. (Gazzaniga and others suspect that these early sections were incomplete, which might also explain why they didn’t help the seizures.) Conversely, studies conducted by Sperry and colleagues in the 1950s revealed greatly altered brain function in animals that had undergone callosal sections.

So WJ underwent the surgery, hoping to get rid of his medical problems. Little did he know that he would end up being the subject of one of the most interesting experiments in psychology to have ever been conducted.

The science

Your brain is split into 2 halves, or hemispheres. The left hemisphere controls the right half of your body, and vice versa. However, in order to function normally, the two halves need to communicate with each other. This happens through the corpus callosum.

However, as explained above, this allowed generalised seizures to occur by allowing neurons in both sides of the brain to fire simultaneously. In order to prevent this, the corpus callosum can be cut, preventing neurons in the opposite halves from communicating. This has an interesting effect: The two hemispheres still control their respective halves of the body, but without communication with each other. This means that anything that only left brain percieves, right brain will have no knowledge of.

A good example of this would be holding something in your left hand and only seeing it through your left visual field. A split-brain patient would not be able to say aloud the name of the object, because the speech center of the brain is in the left hemisphere, but only right brain can actually see the object.

The patient was shown two pictures, one exclusively to the left hemisphere and one exclusively to the right, and was asked to choose from an array of pictures placed in full view in front of him those that were associated with the pictures lateralized to the left and right brain. In one example of this kind of test, a picture of a chicken claw was flashed to the left hemisphere and a picture of a snow scene to the right hemisphere. Of the array of pictures placed in front of the subject, the obviously correct association is a chicken for the chicken claw and a shovel for the snow scene. Patient P.S. responded by choosing the shovel with the left hand and the chicken with the right. When asked why he chose these items, his left hemisphere replied 'Oh, that's simple. The chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed'.


Some people are born without most or all of their corpus callosum, in a phenomenon known as agenesis of the corpus callosum (ACC).

Agenesis of the corpus callosum (ACC) is a brain disorder in which the tissue that connects the left and right sides of the brain (its hemispheres) is partially or completely missing. It is caused by a disruption of brain cell migration during fetal development.

Dr Strangelove

One of the symptoms of split-brain syndrome can be alien hand syndrome, also known as Dr Strangelove syndrome, in which a person's hands can act independently of each other. This makes sense, however it also showed something interesting.

Although it forms during pregnancy, the corpus callosum doesn't come out fully developed from birth. It develops over the course of a person's childhood. During its early stages of development, particularly before speech develops, both halves of the brain are used equally, for whatever things babies use their brain for. However, after speech develops, and the person becomes more social, the left hemisphere, which has the speech center of the brain, becomes more dominant. Meanwhile, the right hemisphere becomes more of a mute companion.

When the hemispheres are severed, the right hemisphere can now act independently, which often leads to disagreements between it and the left hemisphere. However, the left hemisphere is dominant, and makes up most of the conscious and expressive thinking. This means that any actions that the right hemisphere may take, which are unknown to the left hemisphere, seem "alien", since they are not "conscious" to you. This is why in many cases of alien hand syndrome, especially those caused by split-brain syndrome, it is usually the left hand that is perceived as "alien" or out of control.

The brain finds a way

Although it was determined that the left hemisphere was significantly better at speech than the right hemisphere, the researchers could not rule out that the right hemisphere had no speech capabilities.

It would not be surprising to discover that the patients are capable of a few simple exclamatory remarks, particularly when under emotional stress. The possibility also remains, of course, that speech of some type could be trained into the right hemisphere. Tests aimed at this question, however, would have to be closely scrutinized and controlled.

The reason for this scrutinisation and control is the workaround that many split-brain patients, or rather their "brains", found to the problems caused by split-brain syndrome. This workaround was "cross-cuing", in which the right hemisphere would create cues for the left hemisphere to pick up and correct its response.

At first, after either a red or a green light was flashed to the right hemisphere, the patient would guess the color at a chance level, as might be expected if the speech mechanism is solely represented in the left hemisphere. After a few trials, however, the score improved whenever the examiner allowed a second guess. We soon caught on to the strategy the patient used. If a red light was flashed and the patient by chance guessed red, he would stick with that answer. If the flashed light was red and the patient by chance guessed green, he would frown, shake his head and then say, "Oh no, I meant red." What was happening was that the right hemisphere saw the red light and heard the left hemisphere make the guess "green." Knowing that the answer was wrong, the right hemisphere precipitated a frown and a shake of the head, which in turn cued in the left hemisphere to the fact that the answer was wrong and that it had better correct itself! We have learned that this crosscuing mechanism can become extremely refined. The realization that the neurological patient has various strategies at his command emphasizes how difficult it is to obtain a clear neurological description of a human being with brain damage.


The split-brain experiments show the incredible ability of the human mind to adapt in any situation that it is placed in. They also show that all of those quizzes about being "left-brained" or "right-brained" due to being more "artsy" or "sciency" are completely false. In reality, both hemispheres are equally important to function as a normal human being, and tests have shown that both hemispheres are used equally in most situations, whether you're painting or solving a math problem.

The experiments were an important step to understanding how the brain functions, and a lot of the research done by those researchers in 1962 has had a huge impact on the fields of psychology and neuroscience, and the full extent of the brain's abilities are still being studied to this day.

And in case you're wondering, yes, WJ's epilepsy was cured, and his seizures become less frequent and severe.

Additional Sources

Curious: Split Brain

You Are Two

The Curious Case of the People With Split Brains