The Split Brain in Man

T he brain of the higher animals, in­ cluding man, is a double organ, consisting of right and left hemi­ spheres connected by an isthmus of nerve tissue called the corpus callosum. Some 15 years ago Ronald E. Myers and R. W. Sperry, then at the University of Chicago, made a surprising discovery: When this connection between the two halves of the cerebrum was cut, each hemisphere functioned independently as if it were a complete brain. The phenom­ enon was first investigated in a cat in which not only the brain but also the optic chiasm, the crossover of the optic nerves, was divided, so that visual in­ formation from the left eye was dis­ patched only to the left brain and infor­ mation from the right eye only to the right brain. Working on a problem with one eye, the animal could respond nor­ mally and learn to perform a task; when that eye was covered and the same prob­ lem was presented to the other eye, the animal evinced no recognition of the problem and had to learn it again from the beginning with the other half of the brain. The finding introduced entirely new questions in the study of brain mecha­ nisms. Was the corpus callosum respon­ sible for integration of the operations of the two cerebral hemispheres in the in­ tact brain? Did it serve to keep each hemisphere informed about what was going on in the other? To put the ques­ tion another way, would cutting the cor­ pus callosum literally result in the right hand not knowing what the left was do­ ing? To what extent were the two half­ brains actually independent when they were separated? Could they have sepa­ rate thoughts, even separate emotions? Such questions have been pursued by Sperry and his co-workers in a wide­ ranging series of animal studies at the California Institute of Technology over