Parsing the role of sleep in memory processing

Abstract

It would be nice if we could talk about sleep and memory as if there were only one type of memory and one type of sleep. But this is far from the case. Sleep and memory each comes in many forms, and furthermore, memories can go through multiple forms of post-encoding processing that must be individually addressed. Finally, sleep stages per se do not affect memories. Rather, the neuromodulatory and electrophysiological events that characterize these sleep stages must mediate sleep-dependent memory processing. In this review, we attempt to parse out the relative contributions and interactions of these often frustratingly complex systems.

Figure 1

Sleep and memory. Sleep-dependent memory processing depends on (a) the type of memory being processed, (b) the stage or form of processing, (c) the sleep stage in which processing occurs, (d) the neuromodulatory regulation of that sleep stage, and (e) the unique electrophysiological activities found in specific sleep stages.

Figure 2

Multi-item memory processing. When several related item memories are formed in a brief period of time, sleep can process the memories both individually and as a group. Multi-item processing during sleep can extract a gist representation of the entire set of memories, extrapolate a general rule defining the nature of individual items, and even generate false memories from the gist or rules formulated by the sleeping brain. From Ref. [12••].

Figure 3

Transitive inference. When subjects learn the relationships between a series of items, generalization can include the inferring of a transitive relationship among the items. Thus, if taught to select item A over B, item B over C, item C over D, etc., they will infer by transitivity that they should select A over C and B over E. Sleep selectively enhance the more distant transitive assumptions (bottom row). Such inferences of transitivity need not be correct. In the game rock-scissors-paper, one selects rock over scissors and scissors over paper, but paper over rock. From Ref. [13].