"These past studies of brain plasticity generally focused on gray matter, though, which contains the celebrated little gray cells, or neurons, that permit and create thoughts and memories."

"Less research has looked at white matter, the brain’s wiring. Made up mostly of fat-wrapped nerve fibers known as axons, white matter connects neurons and is essential for brain health. But it can be fragile, thinning and developing small lesions as we age, dilapidations that can be precursors of cognitive decline. Worryingly, it also has been considered relatively static, with little plasticity, or ability to adapt much as our lives change. But Agnieszka Burzynska, a professor of neuroscience and human development at Colorado State University in Fort Collins, suspected that science was underestimating white matter. 'It’s been like the ugly, neglected stepsister' of gray matter, she says, ignored and misjudged.... So, for the new study... they 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.... 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).

Why would dancing and walking produce different results? Burzynska suggests that it might be that these dancers spent time watching instructors rather than moving, so maybe dancing continuously would be as productive of neurogenesis as walking. I couldn't tell if the walking was done outdoors. Maybe the difference is not in the exertion but in the experience of moving through and encountering the real physical world.

And here's something else, as expressed in the most-liked comment over there: "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. Not sure why this article never mentions this aspect."

Yes, walking — and, I find, low-key running — does release and sustain a certain kind of flowing, creative thinking — especially if you don't use earbuds to pipe words or music into your head. 

AFTERTHOUGHT: The post title I'm thinking of too late: White Matter Matters.