In the 1950s, biologists regarded the behavioral aspects of the brain as a mystery. Dr. Kandel began to realize that during his generation that mystery was likely to be at least partially solved. He believed that memory storage, once the exclusive concern of psychologists and psychoanalysts, could be approached through techniques used by biologists. Dr. Kandel selected Aplysia as his first model for detecting any chemical or structural brain changes produced by learning, including initial storage and long-term maintenance of information. Aplysia has only 20,000 nerve cells, which are collected into ganglia. The family of cells involved in a single behavioral act is exceedingly small, making it much easier to see the results of training.

Dr. Kandel's method was simple: He repeatedly subjected Aplysia to a painful stimulus that triggered a withdrawal reflex. He found that repeated stimulation of the withdrawal reflex correlated with synthesis of a protein that physically changed an area of the central nervous system ganglia. From this finding, he concluded that chemical and structural changes at the level of single nerve cells were required for short-term memories to become long-term memories.

Studies by Kandel and others have also shown that behaviors associated with learning and memory in younger persons may cause physical changes in the brain that persist through adulthood. For example, studies of violinists indicate that changes in the size of areas of the brain controlling the fingers are much larger in persons who begin violin training early in life. These changes are not as large in persons who begin violin lessons in their 20s, and the later starters never seem to catch up.

Other research shows that memory loss in mice caused by aging can be reversed by changes in brain chemicals. Although human research on learning and memory is so far limited, Dr. Kandel hopes that his research and that of others may lead to ways of reducing normal, age-related forgetfulness.