Even late in life, the hippocampus continues to grow new neurons, said Dr. Gary Kennedy, director of geriatric psychology at Montefiore Medical Center in New York City.
"It's very, very exciting and is essentially proving a theory that has been around for a while, which is that exercise can promote neurogenesis, or the transformation of neural stem cells in the brain into mature, functioning neurons," Kennedy said. "Exercise seems to enhance it or speed it up."
It's possible that exercise could help ward off not only ordinary mental decline but also dementia, Erickson said.
But questions remain. If three days a week is good, would six days a week be better? Would running be better than walking?
"We really don't have a good answer for that," Erickson said, though his prior research found that older people who walked between six and nine miles a week showed significantly less decrease in brain volume over nine years than couch potatoes, but walking more than that didn't seem to increase brain volume any more.
The researchers also looked at the effect of aerobic exercise on two other regions of the brain, the caudate nucleus, which deteriorates with age and disease, and the thalamus, a more stable brain region, but no changes were evident.
"This suggests the effect is not just a widespread increase in brain mass, but it's relatively specific to the hippocampus," Erickson said.
According to background information in the study, in otherwise healthy adults the hippocampus shrinks by about 1 to 2 percent a year, and the decrease is even more rapid among people with dementia.
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