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The memory function of sleep
Susanne Diekelmann and Jan Born
Abstract | Sleep has been identified as a state that optimizes the consolidation of newly acquired information in memory, depending on the specific conditions of learning and the timing of sleep. Consolidation during sleep promotes both quantitative and qualitative changes of memory representations. Through specific patterns of neuromodulatory activity and electric field potential oscillations, slow-wave sleep (SWS) and rapid eye movement (REM) sleep support system consolidation and synaptic consolidation, respectively. During SWS, slow oscillations, spindles and ripples at minimum cholinergic activity coordinate the re-activation and redistribution of hippocampus-dependent memories to neocortical sites, whereas during REM sleep, local increases in plasticity-related immediate-early gene activity at high cholinergic and theta activity might favour the subsequent synaptic consolidation of memories in the cortex.
Declarative memory
Memories that are accessible to conscious recollection including memories for facts and episodes, for example, learning vocabulary or remembering events. Declarative memories rely on the hippocampus and associated medial temporal lobe structures, together with neocortical regions for long-term storage.
Procedural memory
Memories for skills that result from repeated practice and are not necessarily available for conscious recollection, for example, riding a bike or playing the piano. Procedural memories rely on the striatum and cerebellum, although recent studies indicate that the hippocampus can also be implicated in procedural learning.
Although sleep is a systems-level process that affects the whole organism, its most distinctive features are the loss of behavioural control and consciousness. Among the multiple functions of sleep1, its role in the establishment of memories seems to be particularly important: as it seems to be incompatible with the brains normal processing of stimuli during waking, it might explain the loss of consciousness in sleep. Sleep promotes primarily the consolidation of memory, whereas memory encoding and retrieval take place most effectively during waking. Consolidation refers to a process that transforms new and initially labile memories encoded in the awake state into more stable representations that become integrated into the network of pre-existing long-term memories. Consolidation involves the active re-processing of fresh memories within the neuronal networks that were used for encoding them. It seems to occur most effectively off-line, i.e. during sleep, so that encoding and consolidation cannot disturb...