New interneurons in the adult neocortex: small, sparse, but significant?

During the last decade, the intense study of adult hippocampal neurogenesis has led to several new lines of inquiry in the field of psychiatry. Although it is generally believed that adult mammalian neurogenesis is restricted to the hippocampus and olfactory bulb, a growing number of studies have described new neurons in the adult neocortex in both rodents and nonhuman primates. Interestingly, all of the new neurons observed in these studies have features of interneurons rather than pyramidal cells, the largest neuronal population of the neocortex. In this review, we discuss features of these interneurons that may explain why cortical neurogenesis has been so difficult to detect. In addition, these features suggest ways that production of even a small numbers of new neurons in the adult cortex could make a significant impact on neocortical function.