OVER THE PAST two decades neuroscientist Michael Gazzaniga and his mentor, Nobel Prize winner Roger Sperry, have demonstrated via a series of brilliant and original experiments that each half of the human brain is independently capable of processing different aspects of reality.

As the brain is organized, the left hemisphere controls movement and keeps track of sensation involving the right half of the body. It also is the principal processor of language. The right hemisphere, in addition to controlling the left side of the body, manages such matters as attention to the left side of space, pattern discrimination and facial recognition.

Recently, Gazzaniga has looked further at the varying roles of the two hemispheres and asked why the brain is organized as it is. His conclusions, which constitute The Social Brain, are that our mental lives are less unified than we have heretofore believed. "A confederation of mental systems resides within us. Metaphorically, we humans are more of a sociological entity than a single unified psychological entity. We have a social brain."

For his experiments, Gazzaniga selected subjects who had undergone a surgical transection of the right and left hemisphere, usually to stop the spread of seizures across the corpus callosum, the connecting link between the right and left sides of the brain. With this link surgically cut, seizures can no longer spread from one side of the brain to the other. But in addition, as Gazzaniga's research illustrates, information is no longer shared between hemispheres. This produces some bizarre and philosophically intriguing situations.

For example, in a typical experiment with such a subject the word "walk" is flashed for the briefest instant into the subject's left visual field, which is then picked up by his right brain hemisphere. The language-formulating left hemisphere, meanwhile, doesn't see the word, in fact isn't even aware that "walk" was transmitted. But several seconds later the subject gets up from his chair. When asked where he's going, the patient responds with something like, "I'm going to get a coke."

As an explanation of what has gone on here, Gazzaniga suggests that the subject is responding nonverbally to the stimulus of the word "walk." However, it is only when he is asked about his action, that he becomes conscious that he is "going to get a coke." In Gazzaniga's view, consciousness is equivalent to the language output of the brain's left side.

Instead of a unitary self residing within the brain, Gazzaniga proposes that "the normal brain is organized into modules and that most of these modules are capable of actions, moods and responses." Only one module, however -- the interpreter, located in the left hemisphere -- employs language. This single interpreter, by explaining the various behaviors produced over time by the brain's modules, enables a person to construct a unified theory of self. In the absence of such an interpreter, behavior becomes fragmented and the personality split into subselves which work at cross purposes.

Without the brain's language center to explain ourselves, it is possible that along with the rest of the animal kingdom, we would no longer feel compelled to come up with any explanations at all. One doesn't have to speculate for long about what it would be like to dwell in a world bereft of beliefs, theories, and explanations. Such a situation already exists, as Gazzaniga argues, in our pets, whose "activities proceed without language and with abandon."

GAZZANIGA GOES further to tangle with an age-old question beloved by philosophers. Can thought exist in the absence of language? To explore this problem, Gazzaniga flashes the word "bike" to a split brain patient's right hemisphere. "Draw with your left hand a picture of the word that I flashed," Gazzaniga requests.

Since the left hand is under strict control of the right hemisphere, Gazzaniga is asking for a performance based on the perception of that hemisphere alone. But since the patient's language-processing left hemisphere hasn't seen anything, he can only protest that he cannot draw anything.

"Go ahead and let that left hand try," Gazzaniga prods.

At this point the person stops talking and his left hand picks up a pencil and draws an accurate picture of a bicycle.

Most intriguing of all, however, is the patient's verbal response to his performance. "I don't know why I drew that." In this experiment, thought (the correct reading of a word and the rendering of that word into a figure) occurs without reference to language and, in fact, contrary to the patient's verbal statement.

In this instance the patient says one thing about himself ("I didn't see anything and therefore can't draw anything") while his performance proves just the opposite!

On the basis of such fascinating discoveries, Gazzaniga presents a point of subtle but overriding importance. Since the left hemisphere operates at all times to provide a "reasonable" explanation for our behavior, can we totally trust our own self evaluation of why we act as we do? Gazzaniga's research suggests that we cannot since the left brain at all times "tries to bring order and consistency to its mental modules, its mental constituency."

Self-understanding therefore, depends less on a precisely formulated "explanation" set out in words and concepts than it does on the observations of what one actually does over time. In other words, there are limits to our capacity for "self insight" based on our brain's organization.

The implications of Gazzaniga's research, if accepted, are nothing less than profound. For instance, self-understanding (the goal of psychotherapy) would seem to elude verbal description: each person's left hemisphere interpreter merely provides one explanation among a host of alternatives, none of which can be confidently identified as "the real one."

In our courts, a defendant's testimony may properly come under suspicion not because of any deliberate intention to be self-serving, but rather because any person's "explanation" for his or her behavior is only true to a point. "The data suggest that our mental lives amount to a reconstruction of the independent activities of the many brain systems we all possess."

Such insights and controversial speculations make The Social Brain daringly original, sobering and thoughtful. The book, as Gazzaniga admits, is "not for the timid."