By analyzing the behavior of split-brain patients, Roger W. Sperry and his colleagues demonstrated that the human brain is actually composed of two brains that may function separately, challenging the idea of a single, unified consciousness.
The bilateral nature of the human organism means that many parts of the body come in two identical mirror images. These paired organs, such as the lungs, kidneys, and eyes, generally perform identical functions. It was long assumed that the two halves of the brain likewise have a single function. It therefore came as quite a surprise to scientists to discover not only that the two halves of the brain perform different activities but also that, in certain cases, each half can function independently.
This discovery was made in the early 1960’s in the laboratory of Roger W. Sperry, a neurophysiologist who had trained with the developmental biologist Paul Weiss. Sperry was already well known for having shown that growing nerve fibers are guided to make predetermined connections. In the early 1950’s, he began to research the function of the corpus callosum. This narrow bundle of some 200 million neurons connects the two halves of the brain. With his graduate student, Ronald Myers at the University of Chicago, Sperry cut a small piece of brain tissue from an animal, as well as other brain tissue connecting the two hemispheres. With a patch over the right eye, visual information stimulating the left eye would be able to enter only the left hemisphere of the brain. Myers then taught these animals a simple task.
When a cat was taught to solve a problem with its left eye, the information was stored in the left hemisphere. When the left eye was then covered, the cat, looking through its right eye, was unable to perform the same task. The information obviously could not pass to the right hemisphere when the corpus callosum was cut. The right hemisphere could, however, learn the task itself. It was possible, in fact, to teach each hemisphere of the cat to solve a single problem using two different methods. It was as if the cat had two separate brains in its skull, each of which could function independently when separated from the other.
Over the next several years, Myers and Sperry also showed that the brains of monkeys and chimpanzees are made up of two hemispheres that can be made to work independently. There was reason to doubt that these findings would be relevant to human brains. The corpus callosum had already been cut in a number of humans in a medical study by the neurosurgeons W. P. van Wagenen
Sperry decided to conduct more careful studies of such split-brain patients. The surgery was performed by Phillip Vogel and Joseph Bogen at the White Memorial Hospital in California. Their first patient was known by the initials W. J., a forty-eight-year-old war veteran who had developed serious seizures. As expected, the surgery significantly improved his condition and did not seem to change his behavior in any obvious way. More careful behavioral studies were begun by Michael S. Gazzaniga, a graduate student in Sperry’s laboratory, which was now relocated to the California Institute of Technology. The experimental procedure was simple: The subject was asked to keep his eyes focused on a point directly in front of him. A picture of some everyday object was then flashed for a fraction of a second to one side of this point, and the subject was asked to report what he saw.
Visual information that enters the brain from the left visual field goes to the right hemisphere, while information from the right visual field is projected to the left hemisphere. A normal person would report having seen an object whichever field it was in. This was not the case for W. J. He accurately reported seeing an object in the right half of his visual field (perceived by the left hemisphere). When a picture was flashed to his left visual field (perceived by the right hemisphere), he denied having seen anything. The right hemisphere was not blind; it simply could not speak. When W. J. was asked to give a manual response by pointing to an object he had seen, he indicated that his right hemisphere could, in fact, perceive the object.
Sperry and Gazzaniga thus solved the problem of the elusive function of the corpus callosum. When a normal person sees an object in the left visual field, the right hemisphere, which obtained the information, sends it via the corpus callosum to the left hemisphere, which then can verbalize a response about what the individual saw. The corpus callosum thus allows communication between the two hemispheres.
Although the right hemisphere was initially considered inferior because it lacked the verbal ability of the left hemisphere, subsequent research showed that the right hemisphere can understand the vocabulary of a ten-year-old. Although unable to direct the mouth to speak, the right hemisphere could direct the left hand to move plastic letters so as to spell out the answers to certain questions.
Roger W. Sperry.
As in the animal studies, each half of the human brain could learn new information, respond to questions, and store memories on its own, acting as if unaware of the second half of the brain. The left half of the brain was clearly superior in language skills and speech. It needed to be determined if the right half had other skills. Gazzaniga tested split-brain patients in another test, where they were required to arrange a set of blocks to match a design in a picture. The left hand (guided by the right half of the brain) was superior on this task. The scientists concluded that the right hemisphere is important for spatial skills.
The split-brain patient proved a valuable subject for allowing scientists to study where certain activities are localized in the human brain. The finding that the human brain is actually able to function as two separate brains in these patients raised the question as to the possible lateralization of functions in the normal intact brain. Jerre Levy, another student of Sperry, showed that the right hemisphere seems to be superior on spatial tasks in intact people as well. Particularly convincing is the work of Doreen Kimura, who developed tests to study information processing in normal individuals. She showed that the left hemisphere is better at interpreting verbal information, while the right hemisphere is better at identifying melodies. In other work, she showed that speech is processed differently in male and female brains.
While hemispheric specialization has become an accepted concept, the type of specialization is understood differently by different scientists. Some describe the left hemisphere as perceiving symbolic information, while the right hemisphere perceives nonsymbolic information. Others refer to the left hemisphere as dealing with verbal, logical, and mathematical skills, while the right hemisphere deals with spatial tasks, such as reading maps or remembering faces. Other scientists think of the left hemisphere as seeing things in a sequential, logical manner, while the right hemisphere sees things in a more holistic way.
The idea of some dichotomy in the human mind has existed for centuries. Whether called active-passive, scientific-humanistic, or yin-yang, the human brain seems to appreciate the concept of a dual mind. The research on brain lateralization provided a biological explanation that could support such a duality: The two hemispheres could be thought of as two ends of a continuum. Books and articles began to appear in the popular press in the 1970’s and 1980’s, exhorting their readers to perform exercises to activate the right hemisphere and free the creative impulses supposedly hidden there. Claims were made that these exercises would make one more imaginative, creative, or a better artist or writer. There is not, in fact, any evidence to show that creativity is linked to one hemisphere. Even if it were, however, it is not clear that exercise will make it more efficient.
The acceptance of the idea of hemispheric specialization did have some positive effects in the general population, as people gained a better understanding of nonverbal forms of intelligence. While most intelligence tests measure what are considered left-brain activities, increasing emphasis is being given to measurements and appreciation of other types of intelligence that may be localized to the right brain. Educational practices also were affected by knowledge of the different functions of the brain hemispheres. The idea of nonverbal instruction was appreciated by many teachers, from sports to languages. These subjects, it was argued, could be learned more quickly by forcing the right hemisphere to learn by nonverbal imitation, rather than giving verbal instructions to the left hemisphere. While the scientific validity for this concept is uncertain, this attitude does seem to lead to a well-rounded education that will benefit many.
Sperry’s studies also had some impact on the understanding of certain disorders. Dyslexia, for example, is a disorder of children who have difficulty learning to read. These children tend to show less than the usual right-hemisphere specialization for spatial relations. Although the data are not unequivocal, the idea that reading disorders are biologically based and not the result of children’s misbehavior has led to more flexibility in dealing with these disorders.
The split-brain research raises certain philosophical considerations: Is the split brain also a split mind? Are there two separate consciousnesses in a single individual? Sperry argues that in a normal individual, the corpus callosum maintains an integrated sense of awareness. It is only after the split-brain surgery that two separate consciousnesses emerge. Bogen believes that there is a duality of consciousness in all normal minds, but it is made more apparent by split-brain surgery.
Gazzaniga and his student Joseph LeDoux
Julian Jaynes
Controversy about the meaning of the split-brain research continues long after its original report in the 1960’s. For stimulating this controversy, Sperry was awarded the Nobel Prize in Physiology or Medicine
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Baskin, Yvonne. “Emergence: Roger Sperry.” In The Omni Interviews, edited by Pamela Weintraub. New York: Ticknor & Fields, 1984. In an interview with Omni magazine, Sperry describes his philosophy of consciousness as an emergent entity that is greater than the simple sum of awarenesses shown by the separate left and right hemispheres. -
Gazzaniga, Michael S. The Bisected Brain. New York: Appleton-Century-Crofts, 1970. A useful description of the work conducted by Sperry from his early days in Sperry’s laboratory to the beginning of his independent research program. Contains figures and references. -
_______. The Social Brain: Discovering the Networks of the Mind. New York: Basic Books, 1985. An engaging account of split-brain research, written for general audiences by one of the leaders in the field. Offers personal insights into early work in Sperry’s laboratory. Elaborates Gazzaniga’s theory of brain modularity. His speculations about the implications for anthropology, religion, and social processes are thought-provoking, but highly speculative. -
Segalowitz, Sid J. Two Sides of the Brain: Brain Lateralization Explored. Englewood Cliffs, N.J.: Prentice Hall, 1983. The best general introduction to the field in a concise, easy-to-read volume. Although written for those with no background, the breadth of coverage and references to more technical papers make this useful also for more advanced readers. Contains balanced analyses of the opposing viewpoints of the principal investigators in the field. Includes discussion of differences in brain lateralization between individuals, between men and women, and between monolingual and bilingual people. -
Sperry, Roger. “Some Effects of Disconnecting the Cerebral Hemispheres.” Science 217 (1982): 1223-1226. Lecture delivered by Sperry in 1981, when he received the Nobel Prize. Unlike most of his writing, this is accessible to the layperson. Summarizes his split-brain work and his thoughts about its ramifications. References. -
Stover, David, and Erika Erdmann. A Mind for Tomorrow: Facts, Values, and the Future. Westport, Conn.: Praeger, 2000. Synthesis of Sperry’s work designed to find a solution to the conflict between objectivity and subjectivity in modern life. Delves into Sperry’s work on the split brain, the theory of consciousness he developed from that work, and the potential applications of that theory to modern dilemmas. Bibliographic references and index.
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