Computational Modeling of Adult Neurogenesis

Abstract

The restriction of adult neurogenesis to only a handful of regions of the brain is suggestive of some shared requirement for this dramatic form of structural plasticity. However, a common driver across neurogenic regions has not yet been identified. Computational studies have been invaluable in providing insight into the functional role of new neurons; however, researchers have typically focused on specific scales ranging from abstract neural networks to specific neural systems, most commonly the dentate gyrus area of the hippocampus. These studies have yielded a number of diverse potential functions for new neurons, ranging from an impact on pattern separation to the incorporation of time into episodic memories to enabling the forgetting of old information. This review will summarize these past computational efforts and discuss whether these proposed theoretical functions can be unified into a common rationale for why neurogenesis is required in these unique neural circuits.T he importance of neurogenesis will be ultimately tied to its impact on the computational function of the overall circuit within which it resides. Notably, the two mammalian regions where neurogenesis is typically studied, the olfactory bulb (OB) and the dentate gyrus (DG) region of the hippocampus, have considerably different functions. The OB is a primary sensory region, essentially the equivalent of the processing layers of the retina, providing an initial transformation of raw olfactory inputs to the brain. In contrast, the hippocampus is one of the deepest structures in the brain, residing many layers away from sensory inputs and vital for episodic memory formation.Interestingly, as noted throughout this collection, although OB and DG neurogenesis share some similarities, there are sufficient differences to make it difficult to generalize across the two systems, not the least of which is the recent observation that humans likely lack OB neurogenesis. This is somewhat supported by a comparative perspective. As with mammals, neurogenesis in birds is limited to several structures with certain species, including neurogenesis in well-studied birdsong regions. Vertebrates with arguably less advanced nervous systems, such as reptiles and fish, have more widespread neurogenesis, suggesting that broad evolutionary pressures to attenuate neurogenesis in other areas of the brain led to the relative isolation of the OB and hippocampus as opposed to an evolutionary gain-of-function process common to the OB and DG. This subtle distinction has implications on how far one can hope for a shared function between these areas and, for this reason, it is not surprising that most computational studies investigating

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