The role of experience in memory consolidation

The hippocampus is believed to play a key role in long-term consolidation during sleep. Additionally, hippocampal place cells - pyramidal neurons that fire in discrete locations in the space - have been used as a reliable behavioural correlate to study learning and memory of spatial tasks. To date, most studies investigating memory consolidation focus on recordings from neural data obtained during tasks the subjects have been previously overexposed to. While this strategy guarantees a higher stability of the spatial map encoding for that specific experience, the reality of more naturalistic settings is that both humans and other animals can encounter multiple events of diverse duration and relevance on a daily basis. Yet, it remains unclear how the brain prioritizes and successfully stores multiple novel events. To address this question, we exposed a group of rats to pairs of novel linear tracks across different days. Each day, rats were allowed to run in each track for a different fixed number of laps, and the experience was preceded and followed by a sleep session. We found that the hippocampus was able to discriminate the different spatial maps even for short exposures with unstable place fields. We also observed awake and sleep hippocampal replay of all tracks regardless of the stability of their spatial representations. However, when presented with similar experiences of different duration in the novel tracks, the hippocampus prioritised the consolidation of the longer experience if the spatial representation of the shorter one was still unstable. Finally, we found that both awake hippocampal replay and theta sequences influenced the levels of subsequent sleep replay. These results aim to add further understanding of how experience shapes the encoding of different spatial trajectories, and how offline activity contributes to the consolidation of their memory representations

Experience-driven rate modulation is reinstated during hippocampal replay

Replay, the sequential reactivation within a neuronal ensemble, is a central hippocampal mechanism postulated to drive memory processing. While both rate and place representations are used by hippocampal place cells to encode behavioral episodes, replay has been largely defined by only the latter - based on the fidelity of sequential activity across neighboring place fields. Here we show that dorsal CA1 place cells in rats can modulate their firing rate between replay events of two different contexts. This experience-dependent phenomenon mirrors the same pattern of rate modulation observed during behavior and can be used independently from place information within replay sequences to discriminate between contexts. Our results reveal the existence of two complementary neural representations available for memory processes