Shufflebrain

Laboratory animals display interesting behavior after split-brain surgery. Two disconnected hemispheres may learn to respond to what would otherwise be conflicting stimuli. The animals can even learn at a faster pace. (There are, after all, two intelligences instead of one.) One side of the brain may be taught to avoid a stimulus that the other side responds to favorably. A split-brain monkey, for instance, may lovingly fondle a toy doll with its right hand and angrily beat it with the left. (Arms are voluntarily controlled by opposite hemispheres.) Sperry has even reported that persons with split brains sometimes maintain two entirely different attitudes toward the very same object--simultaneously.

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At first glance, and when the results were new, split-brain research looked like a powerful case for a structural theory of mind-brain. (I used to refer to it in the classroom.) Language memory, for example, seemed to be housed in the dominant hemisphere (along with handedness). Music memories seemed to be stored over on the nondominant side. But as more facts emerged, and as all the evidence was carefully weighed, what seemed like such a clear-cut case became fuzzy again.

As I mentioned earlier, some people are born without a corpus callosum. Sperry's group studied one such young woman extensively.[4] Unlike persons who have undergone split-brain surgery, those born without a corpus callosum don't show lateralization: both hemispheres reveal similar linguistic ability. Children who have had split-brain operations show much less lateralization than adults. A few years ago, after I'd written a couple of feature articles on hemispheric differences, a student who had read one of them came to see me, puzzled. If the left side of the brain stores language, he asked, how do people taking an amobarbital test know the lyrics of a song when only the right hemisphere sings?

It was a perceptive question. Clearly, no natural law confines language to one and only one side of the brain. Otherwise, no one with complete separation of the cerebral hemispheres could handle language on the right side; and children would show the same degree of lateralization as adults. Nor would Bogen and Gordon have found individual variations in music or language during the amobarbital test.

Gazzaniga has conducted a great deal of research on children. Before the age of two or three, they exhibit little if any lateralization. Hemispheric differences develop with maturity. We are not born with lateralized brains. How do most of us end up that way?

Circuitries in the visual system can be altered by the early visual environment.[5] There's direct evidence about this for laboratory animals, and a good circumstantial case has been made for humans. Environment has a much more profound effect on even relatively uncomplicated reflexes than anyone had ever suspected. Maybe culture and learning play critical roles in lateralizing. Maybe as we mature, we unconsciously learn to inhibit the flow of information into one side of the brain or the other. Maybe we train ourselves to repress memories of language in the right hemisphere. Maybe the formation of language and the routines in arithmetic proceed more efficiently when carried out asymmetrically--unless we are singing.

Inability of a right hemisphere to read doesn't necessarily preclude memory there, though. Maybe the right hemisphere has amnesia. Or, relying on the left side to handle language, the right hemisphere may simply not remember how it is done. We do repress and do not remember all sorts of things, all the time. I cannot recall my third-grade teacher's name, although I'll bet I could under hypnosis. With regard to repression, consider something like functional amblyopia, for instance--blindness in an eye after double vision, even when there are no structural defects in the eye. It is as though the mind prefers not to see what is confusing or painful--as double vision can be. But with correction of the double view, that same blind eye sometimes regains 20/20 vision.

Thus, we really cannot turn the results of split-brain research into a conclusive argument in favor of a structural theory of mind. We do not know whether split brains show us the repository or the conduits of memory. We do not know if what is coming out flows directly from the source or from a leak in the plumbing.

But the split human brain raises still another question: What does the operation not do? Why didn't the knife create half-witted individuals? Why were both personalities "just folks," as Gazzaniga said? Why two whole personalities? Isn't personality part of the mind? Why doesn't personality follow the structural symmetry of the brain? If we split this page in two, we wouldn't have whole messages in both halves.