TWO DISTINCT TIMESCALE REPRESENTATIONAL DRIFT OF HIPPOCAMPAL CELL ENSEMBLES DURING POPULATION DRIFT AND ITS RELEVANCE FOR COGNITIVE FLEXIBILITY

Memory representations must remain stable over time while retaining sufficient flexibility to integrate new experiences and temporal dimensions. In the hippocampus, memories are encoded by neuronal ensembles whose stability has long been assumed, yet recent studies indicate that these ensembles progressively reorganize even in stable environments (i.e. representational drift). However, the temporal organization and functional relevance of this drift remain poorly understood. In this longitudinal study, we used calcium imaging in freely behaving mice to investigate the dynamics of representational drift in the CA1 region of the hippocampus at two timescales (within days and over weeks) and its functional relevance for behavior and memory performances. To this end, mice repeatedly explored the same environment across multiple days, with four sessions per day. By tracking individual neurons across sessions, we quantified ensemble reactivation, the recruitment of newly active neurons, changes in population activity and spatial representations. We show that hippocampal ensembles undergo a progressive reorganization at two temporal scales: a rapid intra-day drift over hours and a slower inter-day drift overs days in neuronal reactivation, accompanied by continuous changes in population activity patterns. Decoding analysis reveal that temporal information is structured within population activity, indicating a non-random drift. To assess the functional relevance of these dynamics, cognitive flexibility was evaluated using a radial arm maze task. This behavioral framework probes how dynamic hippocampal representations relate to behavioral flexibility and if they can predict performance, raising the possibility that representational drift contributes to flexible memory updating over time.