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

Abstract

Hippocampal neurons represent multiple dimensions of stimulus and behavioural context, including time. It is unclear how soon time-encoding cells emerge during learning, and if they additionally encode context in a manner specific to behavioural paradigm. We investigated simultaneous time and context encoding using 2-photon calcium imaging of mouse hippocampus during a trace eyeblink conditioning (TEC) task with sound and light stimuli. We find that the fraction of time cells is independent of learning and of stimulus modality. Only 13% of cells retain time-encoding on successive days, but persisters remain active within their original stimulus and post-stimulus epochs. Finally, we show that 60% of modality-specific time-encoding cells are active after the stimulus period, but modality-agnostic time cells are rare post-stimulus. Thus, compared to other paradigms, time cells in TEC have distinct learning and turnover properties, and exhibit sustained coding of stimulus modality and time which may subserve associations with subsequent events.