CA1 pyramidal cells organize an episode by segmented and ordered events

A prevailing view is that the brain represents episodic experience as the continuous moment to moment changes in the experience. Whether the brain also represents the same experience as a sequence of discretely segmented events, is unknown. Here, we report a hippocampal CA1 9chunking code9, tracking an episode as its discrete event subdivisions (9chunks9) and the sequential relationships between them. The chunking code is unaffected by unpredicted variations within the events, reflecting the code9s flexible nature by being organized around events as abstract units. The chunking code changes accordingly when relationships between events are disrupted or modified. The discrete chunking code and continuous spatial code are represented in the same cells, but in an orthogonal manner, and can be independently perturbed. Optogenetic inactivation of MEC inputs to CA1 disrupts the chunking but not spatial code. The chunking code may be fundamental for representing an episode, alongside codes tracking continuous changes.