The hippocampus is essential in the initial encoding and subsequent consolidation and retrieval of long term memories. Both consolidation and retrieval are thought to depend on the reactivation of previously stored patterns of neural activity. Memory consolidation, however, has been primarily associated with sleep and the retrieval of memory has been primarily associated with waking behavior. A new paper “Hippocampal replay in the awake state: a potential substrate for memory consolidation and retrieval” (published February 11, 2011 in Nature Neuroscience) reviews research that has established the replay of memories during awake and behaving animals.

Neurons in the hippocampal formation fire when an animal visits a particular place defined by a small region (firing field) and are known as place cells. An individual place cell may respond to more than one location (firing field) each known as a place field. The sequential reactivation of hippocampal place cells reflects an animal’s memory of movement through an ordered set of place fields that together form a cognitive map of locations the animal has visited.

Much has been made of the reactivation of stored hippocampal representations during sleep since 1) the phenomenon was first observed during sleep and 2) sleep seems like an ideal time for memory consolidation to occur. The main thrust of the current review is to point to current evidence that shows sharp wave ripples and the replay of spatial memories occurs in awake and behaving animals. Replay has been shown to be most prevalent immediately after an experience in the awake animal and to decay with time. Nevertheless, the replay of a spatial memory may persist at above chance levels even 18–24 hours after an experience. Even more surprising, the awake replay of a remote environment has been shown to be a higher fidelity recapitulation of past experiences than when a replay was seen during quiescent, sleep­-like states.

The authors point out that forward replay during both behavior and subsequent sleep seems well suited for consolidation of memories related to experienced trajectories and they speculate that forward replay during behavior may enable the retrieval of future paths to aid memory­ guided decision making. Reverse replay during behavior may link recently experienced sequences to their outcome.


Other related blog posts:

Sensation and Location in the Hippocampal Formation