Adult neurogenesis: optimizing hippocampal function to suit the environment.

Numerous studies have attempted to determine the function of adult neurogenesis in the hippocampus using methods to deplete new neurons and examine changes in behaviors associated with this brain region. This approach has produced a set of findings that, although not entirely consistent, suggest new neurons are associated with improved learning and reduced anxiety. This paper attempts to synthesize some of these findings into a model that proposes adaptive significance to experience-dependent alterations in new neuron formation. We suggest that the modulation of adult neurogenesis, as well as of the microcircuitry associated with new neurons, by experience prepares the hippocampus to meet the specific demands of an environment that is predictably similar to one that existed previously. Reduced neurogenesis that occurs with persistent exposure to a high threat environment produces a hippocampus that is more likely to respond with behavior that maximizes the chance of survival. Conversely, enhanced neurogenesis that occurs with continual exposure to a rewarding environment leads to behavior that optimizes the chances of successful reproduction. The persistence of this form of plasticity throughout adulthood may provide the neural substrate for adaptive responding to both stable and dynamic environmental conditions.