"We are finding that the behaviors that people engage in in return have an effect on the underlying structure of the brain and on the number of cells and how they're wired in the brain, which then will subsequently have an affect on what your next behavior is going to be."

Even clearer proof that the human brain benefits from aerobic exercise was found in a study at the University of Illinois' Beckman Institute for Advanced Science and Technology. Its study, funded by the National Institute on Aging, examined the cognitive impact of walking or doing toning exercises on 124 adults ranging in age from 60 to 75.

Participants in both exercise groups showed improvement doing a repetitive task, such as pushing a button, when given a visual cue. However, the walkers were better able to process and ignore irrelevant cues and successfully complete tasks than were those who had done only toning exercises.

Processing relevant information and discarding distractions are essential to "executive control," a term that covers such things as planning, inhibition and temporarily maintaining information in memory.

"Executive control processes are largely controlled by the frontal and prefrontal regions of the brain, areas which show negative metabolic and morphological changes during the normal aging process," says Arthur F. Kramer, a psychologist and researcher at the Beckman Institute. "Cells shrink and blood flow decreases. The benefits you get from walking are in the varieties of cognition that show the largest age-related decline."

Brain Workout World
The question we are left with is, what should we do to stimulate our personal cognitive development, and when do we know it's working? When you do sit-ups, you expect to develop killer abs. We know that when we run or bike for an hour, we feel either exhausted or invigorated by an endorphin rush. But that wears off quickly. How do we know we are doing enough to help the brain muscle?

And if running is so good, what about sports that also involve more thinking, more teamwork, the kind of social interaction that psychologists have long linked with better mental health of the psychologic kind?

The answers will be a long time coming, but there are some theories, at least. "Social stimulus is often quite effective" in neurofunctioning, says Gage, who thinks other sports that involve social interaction may have advantages for neurogenesis, as long as they are aerobic.