A decade ago, several independent brain studies found that adults continue to grow new neurons, overturning a long-held belief that the number of cells in a grown-up's brain can only move in one direction: down. Many scientists suggested that this continued "neurogenesis"—which mainly happens in a small region called the hippocampus—might explain how people continue to learn and adapt in new environments long after birth. But a new mouse study suggests that learning and memory may not depend on neurogenesis. The research raises questions about both the purpose of those mysterious new brain cells and the neurobiology behind adult learning.
For most lab mice, home is a pretty austere place. Five critters share a plastic cage, only slightly bigger than a loaf of bread, lined with wood chips and holding nothing but food and water. Previous experiments had shown that putting mice in more stimulating environments—much larger cages with cubbyholes, tunnels, and exercise wheels—improved their performance on memory tasks, such as water mazes, and decreased their anxiety when exposed to bright light. Mice raised in the fun cages also "had a big boost in neurogenesis" as adults, according to Columbia neuroscientist Michael Drew, which suggested that neurogenesis may be "the way the brain adapts to increased complexity in its environment."
In the new study, Drew and his colleagues tested this hypothesis by irradiating the small hippocampal region where new neurons form, which is thought to be especially important in spatial learning. The irradiated mice, now unable to grow new neurons, were then placed in stimulating cages for six weeks and tested for improved memory and decreased anxiety.
Despite the fact that the mice couldn't grow new neurons, they still fared better on learning tasks than non-irradiated mice raised in cages fit for a prisoner. "We were actually surprised by the results," Drew said. "We fully expected that blocking neurogenesis would block enrichment learning." Since these results suggest new neurons aren't part of the story, Drew says there may be different mechanisms behind enrichment learning, such as a boost in brain proteins that promote connections between existing neurons.
So if the new neurons aren't necessary for these learning tasks, just what is their purpose? Drew says his future experiments will aim to figure out the psychological role of neurogenesis in the hippocampus. "There's a lot of speculation," he says, "that it's involved in depression or the response to antidepressants." After this latest study, current theories on neurogenesis seem to be just that: speculation.