Functional neurogenesis over the years.

There has been interest in the function of adult neurogenesis since its discovery, by Joseph Altman, nearly 60 years ago. While controversy curtailed follow up studies, in the 1990s a second wave of research validated many of Altman's original claims and revealed that factors such as stress and environmental stimulation altered the production of new neurons in the hippocampus. However, only with the advent of tools for manipulating neurogenesis did it become possible to perform causal tests of the function of newborn neurons. Here, we identify approximately 100 studies in which adult neurogenesis was manipulated to study its function. A majority of these studies demonstrate functions for adult neurogenesis in classic hippocampal behaviors such as context learning and spatial memory, as well as emotional behaviors related to stress, anxiety and depression. However, a closer look reveals a number of other, arguably understudied, functions in decision making, temporal association memory, and addiction. In this special issue, we present 16 new studies and review articles that continue to address and clarify the function of adult neurogenesis in behaviors as diverse as memory formation, consolidation and forgetting, pattern separation and discrimination behaviors, addiction, and attention. Reviews of stem cell dynamics and regenerative properties provide insights into the mechanisms by which neurogenesis may be controlled to offset age- and disease-related brain injury. Finally, translation-oriented reviews identify next steps for minimizing the gap between discoveries made in animals and applications for human health. The articles in this issue synthesize and extend what we have learned in the last half century of functional neurogenesis research and identify themes that will define its future.