"Until the late 1990s, most researchers believed human brains were physically fixed and inflexible after early childhood."

"We were born, it was thought, with most of the brain cells we would ever have and could not make more. But... studies using specialized dyes to identify newborn cells indicated that some parts of our brains create neurons deep into adulthood, a process known as neurogenesis... [F]or the new study... [researchers]  divided the volunteers into groups, one of which began a supervised program of stretching and balance training three times a week, to serve as an active control. Another started walking together three times a week, briskly, for about 40 minutes. And the final group took up dancing, meeting three times a week to learn and practice line dances and group choreography..... The walkers and dancers were aerobically fitter, as expected. Even more important, their white matter seemed renewed. In the new scans, the nerve fibers in certain portions of their brains looked larger, and any tissue lesions had shrunk. These desirable alterations were most prevalent among the walkers, who also performed better on memory tests now. The dancers, in general, did not. Meanwhile, the members of the control group, who had not exercised aerobically, showed declining white matter health after the six months, with greater thinning and tattering of their axons and falling cognitive scores."

From "How Walking Can Build Up the Brain/Older men and women who walked for six months showed improvements in white matter and memory, while those who danced or did stretching exercises did not" (NYT).

From the top-rated comment: "One aspect not addressed by this article: While walking, one is engaged in a kind if relaxed thinking. I walk a lot and constantly think about things - important and mundane things. I make plans, solve problems, talk to myself about issues - walking provides a kind of meditative state while you’re doing it."