Hippocampal ensembles represent sequential relationships among an extended sequence of nonspatial events

The hippocampus is critical to the temporal organization of our experiences. Although this fundamental capacity is conserved across modalities and species, its underlying neuronal mechanisms remain unclear. Here we recorded hippocampal activity as rats remembered an extended sequence of nonspatial events and, using novel statistical methods, uncovered new forms of sequential organization in ensemble activity important for order memory judgments. Specifically, we discovered that hippocampal ensembles provide significant temporal coding throughout nonspatial event sequences, differentiate distinct types of task-critical information sequentially within events, exhibit theta-associated replay of the sequential relationships among events, and can represent past, present and future events within individual theta cycles. These findings suggest a fundamental function of the hippocampal network is to encode, preserve, and predict the sequential order of experiences.