Memory reactivation and consolidation during sleep
Abstract
Do our memories remain static during sleep, or do they change? We argue here that memory change is not only a natural result of sleep cognition, but further, that such change constitutes a fundamental characteristic of declarative memories. In general, declarative memories change due to retrieval events at various times after initial learning and due to the formation and elaboration of associations with other memories, including memories formed after the initial learning episode. We propose that declarative memories change both during waking and during sleep, and that such change contributes to enhancing binding of the distinct representational components of some memories, and thus to a gradual process of cross-cortical consolidation. As a result of this special form of consolidation, declarative memories can become more cohesive and also more thoroughly integrated with other stored information. Further benefits of this memory reprocessing can include developing complex networks of interrelated memories, aligning memories with long-term strategies and goals, and generating insights based on novel combinations of memory fragments. A variety of research findings are consistent with the hypothesis that cross-cortical consolidation can progress during sleep, although further support is needed, and we suggest some potentially fruitful research directions. Determining how processing during sleep can facilitate memory storage will be an exciting focus of research in the coming years.
Footnotes
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↵2 REM sleep is a stage of sleep characterized by abundant rapid eye movements, high-frequency EEG signals similar to those during alert waking periods, and other polysomnographic indicators. Dream reports are often elicited upon awakening from REM sleep, but dreams and other sorts of sleep mentation also occur during non-REM sleep. The deepest state of non-REM sleep is known as slow-wave sleep and is characterized by low-frequency (0.5-2 Hz) EEG oscillations. A progression of sleep stages generally occurs in cycles roughly 90 min in length, with progressively longer REM periods over the course of a night's sleep.
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↵3 In both the spiny anteater (echidna) and the platypus, it is likely that REM sleep is absent altogether (T. Allison, pers. comm., July 2004), whereas slow-wave sleep is present, which has been taken to imply that slow-wave sleep evolved much earlier in mammalian evolution than did REM sleep (Allison et al. 1972).
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↵4 We set aside the special circumstances of “lucid dreaming,” when an individual becomes aware that a dream is unfolding and may orchestrate subsequent dream content (LaBerge 1990; Varela 1997; Cicogna and Bosinelli 2001). Future investigations are needed to explore possible ramifications of controlling one's dreams on memory consolidation.
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Article and publication are at http://www.learnmem.org/cgi/doi/10.1101/lm.75704.
- Cold Spring Harbor Laboratory Press
