Episodic-like memory trace in awake replay of hippocampal place cell activity sequences

What is it about?

Episodic memory retrieval of what happened where and when is effective for future planning. Sequential reactivation of the hippocampal place cells along familiar paths while the animal pauses is well suited to such a memory retrieval process. It is however unknown whether this awake replay represents events occurring along the path. This paper examines if not only “where and when” but also “what” information is contained in this awake replay.

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Why is it important?

I found that: (1) awake replay can represent information on subtask as well as path; and (2) the spatial path and non-spatial subtask information were independently encoded in awake replay by temporally compressed versions of global and rate remapping. These results provide a significant conceptual step forward—“what” information of episodic memory is contained in the awake replay of place-cell activity sequences—and advances the view of awake replay as an episodic memory retrieval process. Further conclusions are that: (3) the replay is enhanced by spatial working memory demand; and (4) replay can accurately represent a trial type (i.e. both journey and task demand) to be rewarded when the trial type can be identified based on external cues, but otherwise it evenly represents all possible trial types. In this experiment, trial type can be considered as an episode of what happened where and when. Thus, this result supports the view that awake replay is an episodic-like memory retrieval process.

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