Event:

A prevailing notion in memory research is that the persistence of a memory depends on the stability of the neural codes established during learning. However, recent studies have shown that even in a fixed, familiar environment, hippocampal codes gradually change over timescales of days to weeks - a phenomenon known as representational drift. I will present results from experiments using longitudinal optical imaging of large populations of neurons to study hippocampal and entorhinal codes for long-term spatial memory. While both the passage of time and the amount of experience are known to profoundly affect memory, it has remained unclear to what extent these factors drive representational drift. We found that time and experience differentially influence distinct aspects of hippocampal representations: the passage of time primarily altered neuronal activity rates, whereas experience drove changes in cells’ spatial tuning. Although representational drift in hippocampal place cells is well established, its presence in upstream entorhinal grid codes has remained unexplored. In the second part of my talk I will show that, like hippocampal place codes, entorhinal grid codes also drift over time. Unlike place cells, grid cells (within a module) drift coherently, reflecting shifts in their anchoring to the external reference frame. These findings inform current theories of memory and spatial cognition.