Comparative studies of postnatal neurogenesis and learning: a critical review

In this review we summarize studies of adult neurogenesis, from original de scriptive accounts to more recent work aimed at identifying factors that in fluence neurogenesis as well as its possible function. Much of the recent w ork has focused on the relationship between neurogenesis and learning: we t herefore present critical analyses of specific experiments and the conclusi ons that can be drawn from them. We describe two avian models, song learnin g and food caching, and argue that because these models have a high degree of ethological validity they are particularly well-suited for investigating the fate and specific functions of adult-born neurons. We also describe wo rk on neurogenesis in juveniles and how that has contributed to our underst anding of the relationship between neurogenesis and learning, but caution a gainst drawing inferences regarding the function of adult neurogenesis from these studies in juveniles because the cellular and functional mechanisms underlying juvenile and adult neurogenesis; are likely distinct. We critica lly review several experiments investigating mammalian postnatal neurogenes is, and describe how particular flaws in the assumptions underlying their e xperimental design may render some of their results misleading. We suggest that by increasing the ethological validity of the mammalian models, it may be possible to strengthen the conclusions that can be drawn from such inve stigations. We conclude that there is inadequate empirical evidence linking either the phenomenon of adult neurogenesis with a specific role in learni ng, or learning-associated paradigms with an increase in or enhancement of adult neurogenesis, and indeed, substantial evidence counter to these hypot heses.