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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.