In her invited address at APA's 2002 Annual Convention, where she gave the Neal Miller Lecture, Princeton psychologist Elizabeth Gould, PhD, described how her research has questioned a 100-year-old dogma: The idea that adult mammals are unable to grow new neurons. Gould's research suggests that adult macaques grow new neurons not only in subcortical areas like the hippocampus, but also in the neocortex--the seat of language and other higher cognitive functions.
Since her results were first reported in 1999, she said, the neocortical findings have proven to be controversial, but the overall conclusion--that adult mammals can grow new neurons in the hippocampus and possibly elsewhere--has become widely accepted. "Now [neurogenesis] is under intense investigation by several groups, and we're starting to a get a feel for what happens and under what conditions it occurs," Gould explained.
In the rat hippocampus, "about 9,000 new cells are produced every day, and the majority of these cells differentiate into neurons," said Gould. "This extrapolates to over a quarter of a million new cells every month, which is pretty remarkable given the fact that estimates of the total numbers of this type of neuron in the hippocampus in adulthood range from 1.5 to 2 million."
She added that although scientists don't have good quantitative data for hippocampal neurogenesis in other species, "we know that the phenomenon at least occurs in virtually every mammalian species that's been examined." Gould's own research has focused on the effects of stress and environmental complexity on the growth and survival of new neurons. She has found that stress--and the adrenal steroids it releases--can limit the growth of new neurons.
She and other researchers also have discovered factors that increase neurogenesis. Birds in the wild grow more new neurons than birds in the laboratory. Rats that interact socially with other rats grow more new neurons than rats isolated in standard cages. Complex environments appear to promote neurogenesis; deprived environments appear to inhibit it.
"We feel that we've been studying a phenomenon--adult neurogenesis--that is very, very sensitive to experience, and we've been studying it in animals that are living in such deprived conditions that our data on the lifespan of the new cells, and the number of new cells that are generated, is probably confounded by the manner in which we're housing these animals," said Gould.
To address those concerns, Gould has studied rats in artificial burrow systems where neurogenesis takes place at a higher rate than in standard cages. She has also established in her laboratory a seminaturalistic colony of marmosets, a species of monkey in which she hopes to explore the effect of experience on primate neurogenesis.
"We're really at the very early stages of understanding what, if anything, these new cells do," said Gould. "I think the fact that there are so many neurons that are produced--particularly in the hippocampus--every day, suggests that they must play some important function, because it wouldn't make sense for the brain to expend so much energy to make these new cells if they're not going to be used.
"But we really have a long road ahead of us, and we're just beginning to understand the ways in which we ought to be studying this," she concluded.