One of the more surprising breakthroughs in the 1960s occurred when a scientist realized that the two halves of the brain perform differently and can function independently. Many paired body parts, such as the eyes, kidneys, and lungs, duplicate specific functions; therefore, it is usually possible to live normally with only one of these parts. For example, if a person closes his left eye he can still see with the right eye, and if a person has a kidney removed, the remaining kidney performs for both. Nevertheless, neurophysiologist Roger W. Sperry (1913-1994) was astounded to discover that the two halves of the brain perform different activities, and each half can function independently.

Sperry had been working at the University of Chicago with developmental biologist Paul Weiss, and in the 1950s, he had been studying the corpus callosum, the narrow bundle of some 200 million neurons that connect...