Trends in Cognitive Sciences
ReviewFeature ReviewAdult neurogenesis: integrating theories and separating functions
Section snippets
The challenge of new neurons
Although the nervous systems of other vertebrates exhibit varying degrees of widespread neurogenesis 1, 2, in mammals neurogenesis almost completely ceases after development, with only two regions retaining an ongoing incorporation of new neurons throughout life [3]. One of these regions, the olfactory bulb, is populated by neurons that were originally born in the sub-ventricular zone (SVZ). Immature neurons migrate from the SVZ and give rise to several local interneuron populations in the
The function of the DG in hippocampal processing
Despite its large number of neurons and key position in the hippocampal formation (Figure 1), the DG has not been investigated as extensively as the other principal hippocampal areas, the CA3 and CA1. Nevertheless, several functions have been proposed for the DG 15, 16, most prominently that it is responsible for the pattern separation of cortical inputs to the hippocampus. The separation, or decorrelation, of encoding events is believed to be important to avoid interference between memories
Direct functions of immature, adult-born neurons
New neurons do not arise from existing neurons but rather grow into the adult circuitry de novo, arising from a population of NPCs that reside in the subgranular zone (SGZ) of the DG [34]. Newly born neurons are entirely distinct from the mature DG neurons and must undergo a considerable growth process before becoming recognizable as GCs. While the maturation process is vital to the growing neurons’ survival, and shapes their ultimate characteristics, the properties of neurons at different
Long-term functions of adult-born neurons
Although the evidence regarding new neuron maturation is consistent with the theoretical effects of immature neurons on pattern separation described above, these functions alone are not sufficient to explain the lifelong persistence of adult-born neurons. While a fraction of the immature neurons that contribute to the aforementioned functions probably die before fully integrating into the network, the majority of these neurons seem to survive and eventually attain anatomical and physiological
Conclusions and future directions
This review seeks to link the current theoretical views on neurogenesis function with relevant experimental findings. These different approaches increasingly suggest that the role of neurogenesis is highly complex and affects multiple aspects of learning, as opposed to being a clearly definable function. This subtlety is potentially responsible for the large variance observed between neurogenesis behavioral studies (see [6] for more extensive discussion), and it will continue to present a
Acknowledgements
We would like to thank M.L. Gage for editorial comments on the manuscript. This work is funded by the James S. McDonnell Foundation, the Lookout Fund, the Kavli Institute for Brain and Mind, the NSF Temporal Dynamics of Learning Center, and the National Institutes of Health (MH-090258).
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Contributed equally to this work.