Deep in the bowels of Green Hall, professors aren't the only ones making noise. The monkeys are just as loud.
In elaborate cages packed with spinach and other greens, colorful obstacles and fruit-filled logs, a colony of 44 marmosets — a type of New World monkey — are living under intense scrutiny.
What they, and other animals like them, have shown has overturned one of the central dogmas of neuroscience by disproving the idea that mammals are born with all the neurons they will ever have.
Innovative studies
For more than a decade, psychology professor Elizabeth Gould has conducted dozens of innovative studies in rats and monkeys, demonstrating that new neurons are constantly produced in the adult brains of mammals, including primates.
Her work may have wide-reaching significance, from better understanding how we store memories and react to anxiety and stress to the possibility of repairing damaged brain parts.
To get to this stage, though, it is necessary to start with monkeys.
A group of small primates native to South America, marmosets have a characteristic that make them particularly interesting to study. Like only a few species of mammal, they engage in cooperative breeding: males and older siblings help the mother to parent the babies.
Psychology of parenting
Gould, who came to Princeton in 1997 from Rockefeller University, is exploiting this parenting behavior to examine how experience can induce changes in the adult brain, such as the generation of new neurons or new connections between existing neurons.
"We're looking at what changes in the brain when animals learn how to be better fathers," she said. "This area is so open; no one really knows what goes on in the brains of primates in terms of learning in naturalistic environments."
"Our approach is that the adult brain, and not just the developing brain, can change its structure," she added. "We're trying to understand how this structural plasticity underlies behavioral change."
In addition to parenting, another method used by Gould to study changes in the adult brain involves the complexity of the animals' environment.
In a recent experiment, for example, Gould housed several of the marmosets in standard cages, and some in larger "enriched environments," complete with hidden food and objects that changed every other day.
Cognitive processes
After four weeks, Gould compared the brains of the monkeys from the two environments and found dramatic results. In monkeys who lived in the complex setting, more new neurons were created in the hippocampus and frontal cortex, areas essential for memory and higher-order cognitive processes.
"It seems that living under these conditions in adulthood can change the structure of the brain," Gould said. "So perhaps the control animals are deprived."
Gould added that this finding calls into question conventional laboratory experiments, which may rely on animals whose brains are abnormal because of the simple environment.
Such experiments also minimize individual differences among the animals, making the data difficult to extrapolate to humans, Gould said.
'Growth of new neurons'
Psychology professor Bart Hoebel said what he finds most interesting in Gould's work is the finding that experience can influence changes in the brain.
"It's been shown now that how you use your brain influences connections in existing neurons and also facilitates growth of new neurons," he said.
"Contrast this with recent results showing that some drugs, like ecstasy, kill neurons and reduce neural connections. So in the recent science fair, a lot of my message to the high school students was that you can either get dumber with drugs of abuse, or get smarter with learning."
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