"The pleasure and honor that my right brain feels is more than my left brain can express," said Nobel laureate Dr. Roger Sperry, a pioneering specialist in split brain research, upon receiving one of the major awards for his work.

Once scientists considered the brain's two hemispheres as mirror images of each other, mere backup systems such as other dual organs -- the kidneys, for example, or even the ears and the eyes. Early on they learned that each half of the brain controlled the opposite side of the body -- damage to the left hemisphere affecting motor functions on the right side, for example.

Gradually, as increasingly sophisticated researchers and their high-technology equipment began to tease out the secrets of the brain, it became apparent that although each hemisphere could, under certain circumstances, pick up functions of the other half, there were provinces that tended to be controlled by one half of the brain or the other.

In general, the left hemisphere is seen as the "symbolic-conceptual" side and the right as the "nonsymbolic-directly perceived" side. Left is logic, science, mathematics; right is intuition, imagination and insight.

But not always.

As neurologist Richard Restak writes in The Brain (Bantam, $24.95), "The human brain has yet to discover this aspect of the mystery of its own functioning."

Some findings he cites that so far defy logical explanation:

*Approximately 60 percent of left-handers process speech in the left hemisphere, just as right-handers do. The other 40 percent seem to use both sides.

*A Japanese specialist in hearing and speech disorders has discovered that Japanese process isolated vowel sounds in the left hemisphere, the opposite of virtually every other language group. He speculates, Restak writes, "that the Japanese preference for indirect, intuitive routes to problem-solving may result from the fact that both capabilities reside within the same hemisphere." This also raises speculation that it is the language the infant learns that determines what mental functions are processed in which hemisphere, not something that is predetermined.