Sleep and the extraction of hidden regularities: A systematic review and the importance of temporal rules

Abstract

As part of its role in memory consolidation, sleep has been repeatedly identified as critical for the extraction of regularities from wake experiences. However, many null results have been published as well, with no clear consensus emerging regarding the conditions that yield this sleep effect. Here, we systematically review the role of sleep in the extraction of hidden regularities, specifically those involving associative relations embedded in newly learned information. We found that the specific behavioral task used in a study had far more impact on whether a sleep effect was discovered than either the category of the cognitive processes targeted, or the particular experimental design employed. One emerging pattern, however, was that the explicit detection of hidden rules is more likely to happen when the rules are of a temporal nature (i.e., event A at time t predicts a later event B) than when they are non-temporal. We discuss this temporal rule sensitivity in reference to the compressed memory replay occurring in the hippocampus during slow-wave-sleep, and compare this effect to what happens when the extraction of regularities depends on prior knowledge and relies on structures other than the hippocampus.

Keywords: Memory consolidation; NRT; REM; Regularities extraction; Rule learning; SRTT; SWS; Sleep.

Figure 1:

Comparison of studies. Note that for studies that included several relevant conditions leading to differentiated sleep effects, each condition was treated as a separate study for the purpose of summary statistics as long as different sets of subjects were used in each condition. A: Replication of results by task, for implicit (“Im.”) and explicit (“Ex.”) effects. Only tasks that were repeated in at least two studies are presented. Each icon represents one study; green icons for studies reporting a sleep effect and red icons for null effects. ASRT, Alternating Serial Response Time task; NRT, Number Reduction Task; SRTT, Serial Reaction Time Task; WPT, Weather Prediction Task. Refer to Table 1 for more information about the tasks. B: Comparison between study designs in terms of likelihood of finding a sleep effect. Y-axis: Ratio between the number of studies finding a sleep effect out of the total number of studies with the corresponding design. The specific numbers are indicated above each bar. Designs compared: Time of sleep (overnight/nap); Age of participants (adults/children under 12); Experimental design (sleep vs. wake/sleep vs. sleep deprivation/targeted memory reactivation (TMR) during sleep/single sleeping group); Type of hidden regularity (Temporal/Stationary); Type of hidden regularity only in studies that measured explicit effects (Temporal/Stationary). C: Classification of studies based on whether an implicit or explicit effect was found and the type of regularity used. Each circle represents a study. Color of circle indicates whether any effects in the study were associated with specific physiological sleep measurements (green: effects associated to non-REM sleep, particularly SWS or sleep spindles; blue: effects associated with REM sleep; green/blue: relation to both SWS and REM, or transitions between them; grey: no reported association to any physiological sleep measure, either because none were found or because sleep physiology was not recorded). The Explicit category includes studies that either found explicit effects alone, or both implicit and explicit effects; Circles located in between the Explicit and Implicit-only categories represent studies in which an implicit effect was found together with an indirect or partial explicit effect. See text for details.