Comparing Models of Sleep-dependent Memory Consolidation

Research over the last half century has demonstrated that memory is not unified but is instead composed of anatomically and mechanistically distinct processes.1 The greatest dissociation is between memories that depend on the hippocampus versus those that do not. Hippocampal-dependent memories, also called declarative or explicit memories, are memories for people, places, or things; memories that we can consciously manipulate. In contrast, hippocampal-independent memories, also called non-declarative or implicit memories, are memories to which we do not have conscious access, such as procedural and perceptual learning, habits, priming and conditioning. One strategy for gaining insight into this wide range of memory processes is to The enhancement of normal cognition and breakthrough treatments in cognitive disorders require an improved understanding of memory consolidation. Insights into the mechanisms of memory consolidation have been advanced by the incorporation of a quantifiable variable: sleep. Over the past 20 years, a substantial number of studies have shown that memory performance is facilitated after a bout of sleep, compared with the same period of waking, implicating a slow, offline process during sleep that transforms the memory trace into a more robust form through a consolidation process. Until recently, the majority of these studies have examined cognitive tasks that utilize non-declarative, procedural memory (e.g., knowing “how”, learning actions, habits, perceptual and motor skills, and implicit learning) to show enhanced performance above baseline. Recent attention has turned to studying the relationship between sleep and declarative memory, which refers to consciously accessible memories of fact-based information (i.e., knowing “what”, in terms of events, places, and general knowledge) that are dependent on the hippocampus. Although the exact nature of the relationship between sleep and declarative memory consolidation is hotly debated, there is strong emerging evidence for the importance of slow wave sleep. In contrast with the “enhancement” model of procedural memory, there are two declarative memory models; first, the active model, in which memory depends on sleep specifically; and second, the permissive model, which posits a time-dependent, interference-sensitive process that opportunistically seizes any period of dampened hippocampal input to further process prior, learned information. We review the evidence for the active and permissive models and discuss areas of research that would benefit from future studies. Bridging these scientific fields will impact fundamental research in memory, sleep and pharmacology, as well as have relevance for treatment of memory impairments affecting people with mental illness and age-related cognitive decline. Received: Mar 5, 2010 Revised: Apr 25, 2010 Accepted: May 2, 2010