Memory processes during sleep: beyond the standard consolidation theory

Abstract

Two-step theories of memory formation suggest that an initial encoding stage, during which transient neural assemblies are formed in the hippocampus, is followed by a second step called consolidation, which involves re-processing of activity patterns and is associated with an increasing involvement of the neocortex. Several studies in human subjects as well as in animals suggest that memory consolidation occurs predominantly during sleep (standard consolidation model). Alternatively, it has been suggested that consolidation may occur during waking state as well and that the role of sleep is rather to restore encoding capabilities of synaptic connections (synaptic downscaling theory). Here, we review the experimental evidence favoring and challenging these two views and suggest an integrative model of memory consolidation.

Fig. 1

Two-stage theory of memory consolidation. Top during initial encoding, new information is processed (via sensory neocortex) to the hippocampus, where it is transiently stored via fast synaptic plasticity. Bottom during consolidation, the neocortex receives information from the hippocampus for permanent storage of information (figure similar to [9]). Schematic synapses depict learning-related plasticity

Fig. 2

Experimental design and results from a study of memory consolidation during a brief afternoon nap. a Experimental paradigm. b According to the Standard model of memory consolidation, consolidation occurs predominantly during sleep. c According to the alternative view, consolidation may also occur during waking state, but is facilitated after sleep. d–g Reaction times and intracranial event-related potentials (ERPs) within the hippocampus are consistent with the alternative model (modified from [51])