The Same Hippocampal CA1 Population Simultaneously Codes Temporal Information over Multiple Timescales.

It has long been hypothesized that a primary function of the hippocampus is to discover and exploit temporal relationships between events. Previously, it has been reported that sequences of "time cells" in the hippocampus extend for tens of seconds. Other studies have shown that neuronal firing in the hippocampus fluctuates over hours and days. Both of these mechanisms could enable temporal encoding of events over very different timescales. However, thus far, these two classes of phenomena have never been observed simultaneously, which is necessary to ascribe broad-range temporal coding to the hippocampus. Using in vivo calcium imaging in unrestrained mice, we observed sequences of hippocampal neurons that bridged a 10 s delay. Similar sequences were observed over multiple days, but the set of neurons participating in those sequences changed gradually. Thus, the same population of neurons that encodes temporal information over seconds can also be used to distinguish periods of time over much longer timescales. These results unify two previously separate paradigms of temporal processing in the hippocampus that support episodic memory.