Human memory for recent events is believed to undergo reactivation during sleep. This process is thought to be relevant for the consolidation of both individual episodic memories and gist extraction, the formation of generalized memory representations from multiple, related memories. Which kinds of gist are actually enhanced, however, is the subject of less consensus. To address this question, we focused our design on four types of gist: inferential gist relations extracted across non-contiguous events , statistical learning regularities extracted from a series , summary gist a theme abstracted from a temporally contiguous series of items , and category gist characterization of a stimulus at a higher level in the semantic hierarchy.
Sixty-nine participants 30 men, 38 women, and 1 other completed memory encoding tasks addressing these types of gist and corresponding retrieval tasks the same evening, the morning after, and one week later. Inferential gist and statistical learning were retained over a week, whereas memory for associative gist category and summary gist decayed. Higher proportions of REM and more spindles were associated with worse performance in a statistical learning task controlling for time and after one week, respectively.
Our results suggest consolidation processes promote discovery of rules through synthesis of episodes statistical learning and transitive inference , rather than semantic abstraction per se category and summary gist.
This pattern has been observed in a variety of common paradigms, including transitive inference Ellenbogen et al. One perspective on this gradual emergence of gist is that it reflects the qualitative reorganization of memories during sleep, where new memories emerge that were never learned directly Landmann et al. However, not all "gists" stand to benefit from such reorganization.
We therefore asked the question: do common sleep mechanisms underlie extraction of various gists, and do the various gists benefit differentially? Consistent with this "mere reactivation" view, Walker and Stickgold proposed that SWS supports veridical memory consolidation that keeps individual memories distinct. Under this framework, SWS should preferentially amplify gists internal to a stimulus, such as remembering category gist in the example above.
