Independent population coding of the present and the past in prefrontal cortex during learning

Medial prefrontal cortex (mPfC) plays a role in both immediate behaviour and short-term memory. Unknown is whether the present and past are represented simultaneously or separately in mPfC populations. To address this, we analysed mPfC population activity of rats learning rules in a Y-maze, with self-initiated choice trials followed by a self-paced return during the inter-trial interval. Joint mPfC population activity encoded solely present events and actions during the trial, with decoding of the past at chance; conversely, population encoding of the same features in the immediately following inter-trial interval was solely of the past. Despite being contiguous in time, each population orthogonally encoded the present and past of the same events and actions. Consequently, only the population code of the present during the trials, and not the past coding of the inter-trials intervals, was re-activated in subsequent sleep. Our results suggest that representations of the past and present in the mPfC independently contribute to the learning of a new rule.