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Parallel computation in memory-making
The hippocampus plays a central role in the encoding, consolidation, and recall of memories. Consolidation and recall are thought to be executed by the replay of previously acquired memory traces by hippocampal cell assemblies. The hippocampus is thus considered to be the initiator of memory redistribution processes. However, O'Neill et al. now report that the superficial layers of the medial entorhinal cortex show replay events that are independent of hippocampal activity (see the Perspective by Moser and Gardner). Computations in memory systems may thus be organized in a less hierarchical, more parallel way than previously thought.
Abstract
The hippocampus is thought to initiate systems-wide mnemonic processes through the reactivation of previously acquired spatial and episodic memory traces, which can recruit the entorhinal cortex as a first stage of memory redistribution to other brain areas. Hippocampal reactivation occurs during sharp wave–ripples, in which synchronous network firing encodes sequences of places. We investigated the coordination of this replay by recording assembly activity simultaneously in the CA1 region of the hippocampus and superficial layers of the medial entorhinal cortex. We found that entorhinal cell assemblies can replay trajectories independently of the hippocampus and sharp wave–ripples. This suggests that the hippocampus is not the sole initiator of spatial and episodic memory trace reactivation. Memory systems involved in these processes may include nonhierarchical, parallel components.
