Nearly all evidence of adult neurogenesis in mammals is limited to the hippocampus, and no one is sure what those new neurons do, if anything. Until scientists establish that new, functioning nerves also appear in the cerebral cortex, where higher thoughts are processed, Rakic and others remain skeptical that adult neurogenesis makes much difference to actual brain function. "We start life with a lot of uneducated neurons, but at some point they all become college graduates," he says. "With neurogenesis in the cerebral cortex, you would have neurons that never went to elementary school. New cells would erode all your memories. You would give up all you have labored to acquire."
Elizabeth Gould, a Princeton University neurobiologist who found neurogenesis in the marmoset and other adult primates, argues that the new nerve cells must be useful. "I can’t believe that nature would go to all the trouble of creating thousands of new cells a day to no purpose," she says. "The body is not profligate with its resources."
Nottebohm says the aging brain probably has to develop nerve cells to learn new things. "The brain runs out of memory space," he says. "Everyone past 50 knows that. If we remembered everything, we’d be in overload."
With the same zeal he showed when he first confounded the received wisdom, Nottebohm began new work with blackcapped chickadees in the mid-1990s. One of the American bird species to weather the northern winter, chickadees subsist in that season on seeds and other foods they’ve hidden in trees. Nottebohm found that come autumn, the birds grow new cells in a brain center dealing with spatial memory, the capacity to navigate and find things. The added brainpower helps the chickadees pinpoint their hidden troves months later, Nottebohm says.
Such insight wins admiration. "Fernando has always been ahead of everybody," says Gould. "So far ahead that people for a long time were not able to accept his findings as interesting or important. Now they’re coming around."