At the Salk Institute for Biological Studies in La Jolla, Calif., Neurobiologist Fred H. Gage, in collaboration with a Swedish researcher, detected new neurons in brain tissue of five patients who had died of cancer. The cells had been growing at a rate of hundreds per day. The research was later underscored by studies of living epilepsy patients.

Gage and his Salk colleagues reported evidence that exercise helps keep neurons alive and stimulates cell growth. The scientists found that adult mice living in an enriched environment that included running wheels, tunnels, treats and toys not only improved their learning ability, but also doubled the number of new neurons in an area of the brain that involves long-term memory.

At Princeton University, research showed the number of new brain cells produced per day more than doubled (to 7,000) in adult monkeys who regularly participated in exercises that used motor and decision-making skills. The results indicate the possibility that the structure of the adult mammalian brain can be profoundly altered by a stimulating environment.

An analysis by William Shankle, a cognitive neuroscientist at the University of California at Irvine, found that humans double the number of neurons after birth, especially during the first six years of life.

A study by researchers at the Beckman Institute for Advanced Science and Technology at the University of Illinois found that previously sedentary people over age 60 who walk rapidly for 45 minutes three days a week can significantly improve mental-processing abilities that otherwise decline with age.

Neurons flashing signals through the brain undergo a dramatic change as the mind learns behavioral habits, researchers at the Massachusetts Institute of Technology have found.

As rats learned to run a maze, a change occurred in the firing pattern of certain brain neurons. When the rats were starting to learn the maze, most of the brain neurons fired when the rats made their turn. But as the rats became maze veterans, the pattern shifted. Most neurons fired at the beginning and at the end of the exercise.