Did Splitting the Brain Split Consciousness? The Corpus Callosum, W.J.'s Left Hand and the Interpreter, 1940–1981

This is Part 3 of A History of the Science of Consciousness. Last time, the EEG made it possible to tell the level of consciousness, awake or asleep, without asking anyone. But to learn what the new signs meant, researchers still had to wake sleepers and ask them. This time the person being asked was itself divided in two. In the 1940s, to stop epileptic seizures from spreading, surgeons cut the thick bundle of fibers that joins the two halves of the brain, the corpus callosum. The patients showed no obvious change. Then, in 1962, a test that flashed a shape on only one side of a screen produced a patient who said he saw nothing while his left hand pointed to the spot. Only the left half of his brain had been answering in words. Did the silent right half experience anything? The answer came to depend on what kind of response would count as a report. Working from the original papers and the recollections of the people involved, this article sets out what could be measured and what could not.

The story of two minds in one head

The history of this research is usually told like this. In the 1960s the American researchers Roger Sperry and Michael Gazzaniga studied patients whose two cerebral hemispheres had been disconnected, and showed that one head could hold two minds. The left hemisphere handles language and logic; the right handles shapes and intuition. The discovery led to the 1981 Nobel Prize.

The outline is not wrong. But two things are missing. One is that the same operation had been done some twenty years earlier, and at that time the verdict was that nothing changed. The other is that whether there were two minds depended on how the patients were asked, and on which kinds of answer were accepted. The popular idea that the right brain is creative and the left brain logical was later described by a historian of neuroscience as a simplification exaggerated to the point of being wrong. This article runs from the first report in 1940 to the Nobel Prize in 1981.

Cut, and nothing changed: Van Wagenen's operations and Akelaitis's tests, 1940–1945

The corpus callosum is the thick bundle of nerve fibers that joins the right and left halves of the cerebrum. An epileptic seizure grows as an electrical disturbance that starts in one place in the brain and spreads. William Van Wagenen, a neurosurgeon in Rochester, New York, had noticed that in patients with a tumor or bleeding in the corpus callosum, the seizures behaved differently. If the passage between the two halves were cut, perhaps a seizure could be kept from spreading to the other side. In 1940 Van Wagenen and Herren reported the results of cutting the corpus callosum in patients whose epilepsy did not respond to drugs. In all, a little over twenty people had the operation in the 1940s. Most had only part of the corpus callosum cut; only one had both the corpus callosum and a second, smaller connection, the anterior commissure, cut completely.

The patients were examined in detail by Andrew Akelaitis, a physician who worked with Van Wagenen. He looked at their ability to recognize things by sight and touch, to carry out movements with their hands, and to use language. In a series of reports beginning in 1941, Akelaitis concluded that no obvious changes followed the operation. His 1944 paper was titled "A study of gnosis, praxis and language following section of the corpus callosum and anterior commissure." Cutting the largest bundle of fibers in the brain changed nothing. At the time, some researchers even suspected that the corpus callosum did little more than hold the two halves together.

There were exceptions on record. In 1945 Akelaitis reported that in some patients the left hand moved against the right. Later researchers cite cases in which, as the right hand dressed the patient, the left hand pulled the clothes off again. But this record did not change the overall conclusion.

Why was so much missed? Gazzaniga later pointed to tests that were not sensitive enough and to cuts that were incomplete in many patients. The people being tested could move their eyes freely and touch objects with both hands. Whatever they saw or touched then reached both hemispheres almost at once. Ask the whole person, and the answer came back as one person's answer.

What one eye learned, the other half did not know: Myers and Sperry's animals, 1950s

Animal experiments, meanwhile, were giving a very different result. In the 1950s, under Sperry at the University of Chicago, the graduate student Ronald Myers began experiments on cats for his doctoral research. The work later continued in Sperry's laboratory at the California Institute of Technology and was extended to monkeys.

The trick lay in the pathway from the eyes to the brain. Some of the fibers from each eye cross to the other side at the base of the brain. If this crossing, the optic chiasm, is cut down the middle, what one eye sees reaches only the hemisphere on the same side. An animal prepared this way was trained, with one eye covered, to tell two shapes apart. After it had learned, the cover was moved to the other eye, and the animal still told the shapes apart with no trouble. What one hemisphere had learned was being passed to the other.

But if the corpus callosum and the anterior commissure were cut as well as the crossing, things changed. Shown the task through the untrained eye, the animal behaved as if it knew nothing about it. What one half of the brain had learned, the other half did not know. Sperry gave his 1961 review the subtitle "The split brain behaves in many respects like two separate brains."

In people, though, the reported result was that nothing changed. Gazzaniga later recalled that the idea of a left hand not knowing what the right hand was holding seemed absurd when applied to human behavior. Shortly before, a person born without a corpus callosum had visited the laboratory, and nothing out of the ordinary had been found.

"Nothing," he said, as his left hand pointed: the W.J. tests, 1962

In February 1962, at White Memorial Medical Center in Los Angeles, the neurosurgeon Philip Vogel and the resident Joseph Bogen cut the connections between the two hemispheres of one patient. Identified in the papers as W.J., he was a 48-year-old man. In the Second World War he had served as a paratrooper, and after a jump behind enemy lines he had been knocked unconscious by a blow from the butt of a German rifle. He went on to suffer severe, life-threatening seizures. The operation cut not only the corpus callosum but also the anterior commissure and the other connections between the two sides. The two surgeons thought the earlier operations had worked poorly because the cuts were incomplete. After the operation, W.J. had no more generalized convulsions.

In Sperry's laboratory at Caltech, Gazzaniga, who had just started graduate school, had prepared tests even before the operation. Before surgery, both hands and both eyes worked together normally. A few months later W.J., recovered but still in a wheelchair and wearing a helmet to protect him in case a seizure made him fall, came to the laboratory.

The test worked like this. When the eyes are fixed on a point, whatever lies to the left of that point is sent to the right hemisphere, and whatever lies to the right is sent to the left hemisphere. So W.J. was asked to look at a mark in the center of a screen, and a shape was flashed to its right or left for a tenth of a second. It was gone before he could move his eyes to look, so it reached only one hemisphere. With the corpus callosum cut, there was no route to pass it to the other side.

When a square appeared to the right of center, W.J. said it was a box. When a square appeared to the left and he was asked what he had seen, he said, "Nothing." Asked again whether he had really seen nothing, he said, "Nothing." Gazzaniga wrote that his heart was racing. He then flashed small circles on one side or the other and asked W.J. to point to what he saw with whichever hand he liked. When a circle appeared on the right, the right hand rose; when it appeared on the left, the left hand rose, and each pointed to where the circle had been. The right hand, run by the left hemisphere, and the left hand, run by the right hemisphere, had each seen. But only the left hemisphere could say so.

In another test from the same period, W.J. was asked to arrange colored blocks to match a pattern on a card. His right hand could not do it, while his left hand did it easily. At times the left hand even reached in to help the fumbling right one. The results were reported in October 1962 by Gazzaniga, Bogen and Sperry in the Proceedings of the National Academy of Sciences.

W.J.'s "Nothing" was not a lie. The one answering aloud was the left hemisphere, which had not seen the shape. The patient's honest report was the report of half his brain.

Asking the silent right hemisphere: answers by pointing and by spelling, 1968–1977

After W.J., the Caltech group studied others who had the same operation, sixteen people in all. In his 1968 paper and elsewhere, Sperry argued that each disconnected hemisphere seemed to have its own perceptions, learning and memories, of which the other knew nothing. The speaking left hemisphere said it knew nothing of what the right hemisphere had done. The right hemisphere understood spoken and written language to some degree, but it could hardly speak or write at all.

How, then, to ask the silent right hemisphere? Change the way of answering from the mouth to the hand: point to one of several choices, or pick an object out by touch. Later, a special contact lens that let images reach only one hemisphere was used to show photographs to the right hemisphere. According to Sperry, the right hemisphere picked out photographs of the patient, family members, pets and familiar political figures, and reacted with fitting emotion and even a sense of humor.

One right hemisphere "spoke" more directly. It belonged to a teenage boy, recorded as P.S., who had his operation at Dartmouth Medical School on the East Coast. He had had severe convulsions at about age two, and from about age ten his seizures could no longer be controlled; in January 1976 his corpus callosum was cut completely. Unusually, P.S. had far more language in his right hemisphere than any of the other patients the researchers had studied.

Joseph LeDoux, Donald Wilson and Gazzaniga asked their questions this way. A question was read aloud, and one key word in it was flashed for 0.1 to 0.15 seconds on the left side of a screen, so that it reached only the right hemisphere. The answer was spelled out with Scrabble letters, mostly by the left hand. The left hemisphere could not name the word that had been flashed.

The right hemisphere spelled his name correctly. Asked what job it wanted in the future, the right hemisphere spelled "automobile race." Yet the left hemisphere, answering aloud, said again and again that he would be a draftsman. When he rated how much he liked words on a seven-point scale, in the first session the right hemisphere's ratings of sixteen words were consistently worse than the left's, sometimes by several steps. In a session about a month later, the two sides largely agreed. The authors also wrote that on days when the two sides disagreed, P.S. was restless and hard to manage.

In their 1977 paper the three concluded that the right hemisphere, too, had a sense of self, preferences, a sense of the future and goals: a consciousness of its own. They also wrote something else that matters. In other patients whose right hemisphere lacked language, no evidence of consciousness had been found either. Rich language, they suggested, was a reliable sign of the richer side of mental life, and perhaps a necessary condition for it, though the link might also be only apparent.

The left hemisphere explains what it never saw: the interpreter and the Nobel Prize, 1971–1981

With the same patient, Gazzaniga and LeDoux ran another experiment. A chicken claw was shown to the left hemisphere and a snow scene to the right. Then he was asked to choose, from a set of pictures, ones that went with what he had seen. The right hand, run by the left hemisphere, pointed to a chicken; the left hand, run by the right hemisphere, pointed to a snow shovel. Both choices made sense.

Asked why he had chosen those two, the speaking left hemisphere, which had not seen the snow scene, answered at once. The chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed. The left hemisphere did not know the real reason the left hand had picked the shovel. Not knowing, it made up a sensible reason on the spot. In their 1978 book, Gazzaniga and LeDoux called this work of the left hemisphere, which explains one's own actions and ties them into a single story, the "interpreter."

For anyone relying on reports, this was an awkward finding. W.J.'s "Nothing" was a report that said it did not know what it did not know. P.S.'s "you need it for the chicken shed" was a report that spoke of what it did not know as if it did.

Philosophers were puzzled too. In 1971 the American philosopher Thomas Nagel, in his paper "Brain bisection and the unity of consciousness," went through the ways split-brain patients could be read, one by one. There is one mind, and the right hemisphere's responses are not the work of a mind. There are two minds. There is one mind, but poorly integrated. There is normally one mind, which splits in two only during the tests. Nagel argued that none of these readings works, and concluded that there may be no definite answer to the question of how many minds there are.

In 1981 Sperry received half of the Nobel Prize in Physiology or Medicine. The prize was awarded "for his discoveries concerning the functional specialization of the cerebral hemispheres." It does not mention the discovery of two consciousnesses. On 8 December his Nobel lecture was read on his behalf by a member of the prize committee, because Sperry was unwell.

What could be measured at this point, and what could not

What could be measured was that each hemisphere could see, learn and choose on its own. A shape flashed on one side of the screen, or an object felt with one hand, stayed in one hemisphere once the corpus callosum was cut. That showed up as differences anyone could check from outside: whether the patient could say it, and which hand could point to it. It also became clear that, in most people, speech comes from the left hemisphere. And it turned out that the speaking left hemisphere will supply reasons for things it does not know.

What could not be measured was whether the silent right hemisphere experienced anything. Its answers were behavior: pointing, choosing, arranging letters. Researchers judged whether there was consciousness by whether they accepted that behavior as a report. In P.S., whose right hemisphere could use language, it was said to have "a consciousness of its own"; in right hemispheres without language, there was "no evidence of consciousness." The answer to how many minds there were depended on which kinds of answer were counted as reports. And even the spoken reports of the left hemisphere could not be taken at face value, because it gave reasons for things it had never seen, without hesitation.

The way of counting itself later wavered. Sperry himself acknowledged in his 1968 paper that even among the few patients studied there were striking differences and outright exceptions. In 2017 a report described two patients whose corpus callosum had been cut completely and who could show where something had appeared, on either side of the screen, by speaking or with either hand. Those researchers argued that what had been divided was perception, not consciousness. The answer is still not settled.

In Part 1, behaviorism set reports aside. In Part 2, reports came back to give the EEG its meaning. With the split brain, the one doing the reporting was itself divided in two. One half spoke, the other pointed, and the speaking half gave reasons it did not have. Here the question of how far to trust a report became a question of whose report it was.

Next time: cases where the brain and body answered first, while the person was saying "I can't see it" or "I haven't decided yet." We follow blindsight, in which patients who had lost part of their field of vision located a light they said they could not see, and Libet's experiment, in which a change in the brain's electrical activity began before people felt they had decided to move (1965–2012).