Time cells emerge early in learning and encode stimulus modality past task requirements

Abstract

Hippocampal cells represent multiple dimensions of stimulus and behavioural context. Time-cells encode the dimension of time between events, but it is unclear if their properties depend on the behavioural paradigm. We find that in the sub-second time-scales of trace eyeblink conditioning (TEC), time cells occur both in stimulus and post-stimulus periods, and are insensitive both to modality and to state of learning. This contrasts with time-cell stimulus-dependence in delay non-match to sample (DNMS) tasks (~10 seconds). We observe 87% turnover of time-cells on successive days, but those time-cells which persist retain selectivity to stimulus and post-stimulus periods respectively. Finally, we show that 60% of time-encoding cells are active after the stimulus period, potentially providing an event trace for the network to link to later stimuli. Overall, TEC time-cells have distinct properties during turnover and are independent from behavioural state and contingency compared to DNMS.