Procedure used to study the activities of the two hemispheres of the brain separately, and independent of each other.
Psychologists have demonstrated that even simple human tasks, like thinking of a word when viewing an object, involve separate subtasks within the brain. These smaller tasks involve identifying the object, assessing its use, remembering what other objects are related to it, determining how many syllables are in the word associated with the object, and so on.
People do not realize the complexity of seemingly simple tasks because the brain integrates information smoothly and flawlessly almost all the time. One structure in the brain involved in the exchange and integration of information from one part to the next is the corpus callosum, a bundle of about 200 million nerve fibers that connect the right and left hemispheres of the brain.
Beginning in the 1940s, neurologists questioned whether the corpus callosum was involved in the development of epileptic seizures. Evidence from monkeys suggested that abnormal neural responses in one hemisphere spread to the other via the corpus callosum, resulting in major seizure activity. As such, it might be beneficial to patients suffering from epilepsy to sever the corpus callosum in order to prevent the spread of this abnormal neural activity. After some initial problems with the surgical procedure, neurologists documented the benefit of such surgery, called cerebral commisurotomy. This so-called split-brain surgery resulted in an increase in split-brain research. One of the primary researchers in this area was neurosurgeon Roger Sperry (1913-1994).
Research neurosurgeons discovered that after surgery, patients often experienced a short period during which they could not speak and had difficulty controlling the left side of their bodies. This set of problems, called acute disconnection syndrome, probably reflected the trauma caused by the surgery itself. After the patient recovered from this trauma, his or her everyday behavior appeared unchanged. The two hemispheres of the brain were no longer directly connected, so information from one half of the brain should not have been able to get to the other. Researchers required subtle and sophisticated techniques to be able to differentiate people whose corpus callosum had been cut from those with intact brains.
Such techniques involve using apparatus that can present visual information so that it goes to only one side of the brain. In this case, split-brain patients may not be able to label a picture that stimulates on the right side of the brain; they may have no difficulty when the left side of the brain, which normally controls language production, receives stimulation. At the same time, research in the area has been conflicting. Some work reveals considerable sophistication in language ability in the right hemisphere. Although language functions do differ across hemispheres, split-brain research has not completely resolved the issue about the nature and the degree to which the left and right hemispheres differ.
More recent research has suggested that the left hemisphere may be involved in much linguistic behavior because of its strength in dealing with analytical, structured tasks. On the other hand, the right hemisphere may be better in spatial tasks because these tasks require holistic, synthetic functioning—the strength of the right hemisphere.
When the patients were asked to point to pictures of the normal faces, they selected the normal face associated with the half of the chimeric face that stimulated the right hemisphere. When forced to respond verbally, the patients showed a preference for the picture that had stimulated the left hemisphere. Although researchers cannot specify the exact differences in the functioning of the two hemispheres, regular differences along visual and linguistic lines have emerged.
Although the research has demonstrated differences in the functioning of the two hemispheres of the brain, everyday behavior may appear completely normal in split-brain patients. This is true because human behavior is very flexible and adaptable. For example, a split-brain patient might turn the head when focusing on an object; thereby stimulating both hemispheres. Further, these patients use cross-cuing in which they invoke as many different modalities, like vision, audition, and touch, to help them make sense of their world.
Springer, S. P., and G. Deutsch. Left Brain, Right Brain. 2d ed. New York: W. H. Freeman, 1985.