Hippocampal replay during waking states

Very cool study by Karlsson and Frank showing that rats' reactivate stored representations of remote experiences during waking replay in the hippocampus. But why is it only with remote rather than local replay? They suggest that novel experiences may play a role in which long lasting neuronal excitability and coordination for the cells during those experiences are generated.

In the discussions section they make mention of the probable dysynchronized neocortical state during awake replay constrasted to the synchronized state during sleep replay. They conclude by explaining that awake replay can reactivate a memory of a past experience even during an ongoing experience, potentially creating associations of multiple events across time.

Here's a snippet from their abstract:

Here, we show that stored representations are reactivated during both waking and sleep replay. We found frequent awake replay of sequences of rat hippocampal place cells from a previous experience. This spatially remote replay was as common as local replay of the current environment and was more robust when the rat had recently been in motion than during extended periods of quiescence. Our results indicate that the hippocampus consistently replays past experiences during brief pauses in waking behavior, suggesting a role for waking replay in memory consolidation and retrieval.

Karlsson MP, & Frank LM (2009). Awake replay of remote experiences in the hippocampus. Nature neuroscience, 12 (7), 913-8 PMID: 19525943

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