Hippocampal neurogenesis and forgetting

•Previous data examined the anterograde impact of hippocampal neurogenesis on memory. •We propose a novel yet complementary retrograde role for neurogenesis in forgetting. •Continued integration of new neurons disrupts patterns of previously stored information. The hippocampus is thought to automatically encode all experience, yet the vast majority of our experiences are not remembered later. Although psychological theories have postulated the existence of decay processes for declarative memory, the corresponding neurobiological mechanisms are unknown. Here we develop the hypothesis that ongoing hippocampal neurogenesis represents a decay process that continually clears memories from the hippocampus. As newborn granule cells integrate into established hippocampal circuits, they form new input and output connections over the course of several weeks. Because successful memory retrieval relies on reinvoking patterns of activity that occurred at the time of encoding (pattern completion), neurogenesis-induced remodeling of hippocampal circuits incrementally reduces the likelihood that a given retrieval cue will reinvoke a previously stored pattern. The hippocampus is thought to automatically encode all experience, yet the vast majority of our experiences are not remembered later. Although psychological theories have postulated the existence of decay processes for declarative memory, the corresponding neurobiological mechanisms are unknown. Here we develop the hypothesis that ongoing hippocampal neurogenesis represents a decay process that continually clears memories from the hippocampus. As newborn granule cells integrate into established hippocampal circuits, they form new input and output connections over the course of several weeks. Because successful memory retrieval relies on reinvoking patterns of activity that occurred at the time of encoding (pattern completion), neurogenesis-induced remodeling of hippocampal circuits incrementally reduces the likelihood that a given retrieval cue will reinvoke a previously stored pattern.