Integrated Calendar

Following initial learning, information stored in memory undergoes a time- and experience-dependent evolution. Currently, the nature of this evolution at the neuronal ensemble level remains largely unknown. To obtain insight into this dynamic process we optically image memory-associated neuronal representations in large populations of single cells over long periods of time. Our work focuses on neural coding in the hippocampus, a brain structure that is important for memory of places and events. I will present new work in which we tracked the activity of large populations of hippocampal pyramidal neurons over weeks, as the mice repeatedly explored different familiar environments. Longitudinal analysis exposed ongoing environment-independent evolution of episodic representations, despite stable place code and constant remapping between the two environments. These dynamics time-stamped experienced events via neuronal ensembles that had cellular composition and activity patterns unique to specific points in time. Temporally close episodes shared a common timestamp regardless of the spatial context in which they occurred. Temporally remote episodes had distinct timestamps, even if they occurred within the same spatial context. I will discuss how these findings relate to current understanding of the role of the hippocampus in long-term episodic memory.