Let us now consider our hemispheres and what they do in more detail. The fact of handedness is an immediate indication that they sense and seem to deal with the world in two different ways. Psychologists, neurologists, and now neuropsychologists have, especially over the last five decades, turned out an enormous amount of research that describes the many subtleties of these differences.

The person most associated with getting this research off the ground is the late Roger H. Sperry of Cal Tech University, who received a Nobel Prize for his work. In 1953, Professor Sperry, then at the University of Chicago, suggested that one of his students, Ronald W. Myers, cut the optic chiasm of a cat to see what would happen. The optic chiasm is the point where the nerve tracts from the two eyes cross in the brain. This crossing allows the impulses from the left of the retina in both eyes to make their way to the left hemisphere and the same with the right half of the eyes to the right hemisphere. With the chiasm cut, each eye sends its sensations to one hemisphere rather than both.

Sperry and Myers covered one eye of this cat and trained the animal to discriminate visually between stimuli for a reward. When they then covered the cat's other eye and exposed it to the same stimuli, the animal responded appropriately. In other words, the animal did not have to be retrained despite the fact that direct visual information had been sent only to one half of its brain. The only way this could have happened was by communication between the hemispheres through the corpus callosum.

This observation led to a second set of experiments where they severed both the optic chiasm and the corpus callosum. Now when they trained the cat with one eye covered there would be no transfer of information from one hemisphere to the other. And, indeed, when after this training they tested the cat with the formerly open eye covered, it could not perform. Reports Michael Gazzaniga, another of Sperry's students, "Here for the first time in the history of experimental psychology, surgical disconnection of a brain structure had resulted in the breakdown of high order 'mental' properties between brain areas."2 From here Sperry and his colleagues continued to experiment with other animals and confirmed the function of the corpus callosum as the nerve tract through which the two hemispheres communicate.

In the thirties and forties, surgeons performed about two dozen commissurotomies (the name given for the sectioning of the corpus callosum) on human beings. They did this to control the spreading electrical brain storms that characterize particular epileptic seizures. These operations were successful, and in follow-up studies researchers detected no significant psychological differences or personality changes. After examining these results, Joseph E. Bogen and Philip Vogel, Los Angeles neurosurgeons, decided the procedure had promise in controlling the severe epilepsy that was plaguing a veteran who suffered brain damage in World War II. Thus in 1960 they operated on this individual with the happy result that the surgery did relieve his suffering. It also provided Roger Sperry, who by then had moved to Cal Tech, with his first opportunity to study hemisphere differences in human beings. During the next decade, doctors performed more such commissurotomies, and Sperry and his colleagues were able to bring these patients to their labs as well.

Studying Lateralization
The unique thing about such split brain patients is that often neither hemisphere is internally damaged; it is only the nerve fibers between them that have been cut. This means that in one person the researcher can see how each hemisphere processes the same phenomena. This has not, however, been the only way to learn about what is called lateralization. From the mid-nineteenth century doctors have been aware that damage to the left hemisphere usually affects speech and that damage to either hemisphere affects the other side of the body. Studying the behavioral effects of the various kinds of brain damage in human beings has been a major source of our knowledge about the hemispheres. In recent years, scientists have devised other methods to test lateralization in non-commissurotomized, non-brain damaged people as well.

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