Event:

Following initial learning, information stored in memory undergoes a time dependent evolution. To obtain insight into this dynamic process there is a need to follow memory representations in large populations of single cells over long periods of time. I will present work in which we used Ca2+ imaging in freely behaving mice to track the activity of thousands of CA1 pyramidal cells over weeks, as the mice repeatedly explored familiar environments. We found that place coding was dynamic, as each day the ensemble representation of each environment involved a unique subset of cells. However, cells in the overlap between the ensembles active at any two time-points retained the same place fields, which sufficed to preserve an accurate spatial representation across weeks. We further found that such days-scale ensemble dynamics was informative about the temporal relationship between different episodes in different environments. I will discuss how these findings relate to current understanding of the role of the hippocampus in long-term episodic memory.