A contextual binding theory of episodic memory: systems consolidation reconsidered

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Abstract

Episodic memory reflects the ability to recollect the temporal and spatial context of past experiences. Episodic memories depend on the hippocampus, but have been proposed to undergo forgetting unless consolidated during off-line periods like sleep to neocortical areas for long-term storage. Here, we propose an alternative to systems consolidation theory — a contextual binding account — in which the hippocampus binds item- and context-related information. We compare this account with behavioral, lesion, neuroimaging and sleep studies of episodic memory, and contend that forgetting is largely due to contextual interference. Accordingly, episodic memory remains dependent on the hippocampus across time, contextual drift produces post-encoding activity, and sleep benefits memory by reducing contextual interference.

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Sleep and the price of plasticity: from synaptic and cellular homeostasis to memory consolidation and integration.

Giulio Tononi, Chiara Cirelli (2014)

Sleep is universal, tightly regulated, and its loss impairs cognition. But why does the brain need to disconnect from the environment for hours every day? The synaptic homeostasis hypothesis (SHY) proposes that sleep is the price the brain pays for plasticity. During a waking episode, learning statistical regularities about the current environment requires strengthening connections throughout the brain. This increases cellular needs for energy and supplies, decreases signal-to-noise ratios, and saturates learning. During sleep, spontaneous activity renormalizes net synaptic strength and restores cellular homeostasis. Activity-dependent down-selection of synapses can also explain the benefits of sleep on memory acquisition, consolidation, and integration. This happens through the offline, comprehensive sampling of statistical regularities incorporated in neuronal circuits over a lifetime. This Perspective considers the rationale and evidence for SHY and points to open issues related to sleep and plasticity. Copyright © 2014 Elsevier Inc. All rights reserved.

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Memory consolidation, retrograde amnesia and the hippocampal complex.

Lynn Nadel, Morris Moscovitch (1997)

Results from recent studies of retrograde amnesia following damage to the hippocampal complex of human and non-human subjects have shown that retrograde amnesia is extensive and can encompass much of a subject's lifetime; the degree of loss may depend upon the type of memory assessed. These and other findings suggest that the hippocampal formation and related structures are involved in certain forms of memory (e.g. autobiographical episodic and spatial memory) for as long as they exist and contribute to the transformation and stabilization of other forms of memory stored elsewhere in the brain.

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The neurobiology of consolidations, or, how stable is the engram?

Yadin Dudai (2004)

Consolidation is the progressive postacquisition stabilization of long-term memory. The term is commonly used to refer to two types of processes: synaptic consolidation, which is accomplished within the first minutes to hours after learning and occurs in all memory systems studied so far; and system consolidation, which takes much longer, and in which memories that are initially dependent upon the hippocampus undergo reorganization and may become hippocampal-independent. The textbook account of consolidation is that for any item in memory, consolidation starts and ends just once. Recently, a heated debate has been revitalized on whether this is indeed the case, or, alternatively, whether memories become labile and must undergo some form of renewed consolidation every time they are activated. This debate focuses attention on fundamental issues concerning the nature of the memory trace, its maturation, persistence, retrievability, and modifiability.