Silent hippocampal engrams shape future memory processing

Abstract

The hippocampus harbors silent memory traces even when memory recall is absent. It is unknown why these traces persist and whether they remain sensitive to future episodes. We trained mice on the hippocampal-dependent novel object place recognition task and tracked the transition of the engram from a latent to a silent phase, which still held a functional memory trace. When mice were exposed to a weaker, modified version of their original memories, silent engrams led to better consolidation of these new events than their latent predecessors. Bidirectional manipulation of the engrams state, as well as artificially prolonging its latent state, successfully reversed its effect on the second memory. The silent engram state consistently displayed higher neuronal reactivation during the second event compared to the latent one, which may explain its preferential effect on its consolidation. Our results reveal previously unknown cognitive and activation dynamics for this behaviorally silent state.