Opinion
Decay happens: the role of active forgetting in memory

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Although the biological bases of forgetting remain obscure, the consensus among cognitive psychologists emphasizes interference processes, rejecting decay in accounting for memory loss. In contrast to this view, recent advances in understanding the neurobiology of long-term memory maintenance lead us to propose that a brain-wide well-regulated decay process, occurring mostly during sleep, systematically removes selected memories. Down-regulation of this decay process can increase the life expectancy of a memory and may eventually prevent its loss. Memory interference usually occurs during certain active processing phases, such as encoding and retrieval, and will be stronger in brain areas with minimal sensory integration and less pattern separation. In areas with efficient pattern separation, such as the hippocampus, interference-driven forgetting will be minimal, and, consequently, decay will cause most forgetting.

Section snippets

Current thinking on forgetting

Forgetting of established long-term memory (see Glossary) may indicate that memory is either physically unavailable (that is, memory is lost) or that it is (temporarily) inaccessible. With some exceptions, theories proposed within the domains of experimental and cognitive psychology often emphasize one type of forgetting over the other [1]. Two explanations for actual, non-pathological memory loss have been proposed, one involving decay of aspects of the memory trace, the other involving

Forgetting in multiple memory systems: hippocampus and neocortex

In agreement with most current views (for a review, see [21]), we suppose that what are generally referred to as explicit memories initially consist of two components: content representations largely dependent on neocortical networks and a spatial-contextual representation dependent on the hippocampus that indirectly links, and serves to index, the dispersed neocortical representations. The list- or story-learning tasks usually employed to study forgetting (predominantly interference) in humans

A role for decay in everyday forgetting

The findings from these studies with amnesic patients also show that new learning of similar material or interpolated activity does not cause complete amnesia in healthy control subjects, and, depending on the material, forgetting due to interference may even be minimal. For example, in one study [34], retention of a word list dropped from 41% immediately after learning to 19% when new learning was followed by ten minutes of exacting cognitive tasks. When memory for a story was tested, however,

Possible molecular pathways of decay-like forgetting

Recent studies on long-term memory maintenance support the notion that hippocampal traces can decay over time. These studies established the constitutively active, atypical protein kinase C isoform M-zeta (PKMζ) as both necessary and sufficient for maintaining long-term potentiation and for sustaining long-term memory in various tasks and brain regions [61]. PKMζ is synthesized upon induction of LTP or during formation of memory [62]. After translation, PKMζ is phosphorylated and then stays

Concluding remarks

In this article, we have suggested that decay-like forgetting is a well-organized neuronal process that systematically removes memories from the hippocampus over time, perhaps preferentially during sleep. This type of forgetting is essential to maintain overall system functionality. Because most of the memories automatically formed during the day are irrelevant, such forgetting will ensure that most of these unwanted and unneeded memories are removed. Understanding decay-like forgetting as a

Acknowledgments

We would like to thank our colleagues Paola V. Migues, Todd Sacktor, and Almut Hupbach for commenting on earlier drafts of our paper. We also would like to thank the two anonymous reviewers of this paper for their detailed and constructive criticism. O.H. and K.N. were supported by CIHR and NSERC; L.N. was supported by NSF, Down Syndrome Research and Treatment Foundation, Research Down Syndrome, Fondation Lejeune, and the Thrasher Research Fund.

Glossary

Active forgetting
the idea that, instead of passively disintegrating, memories are actively removed, on the basis of, for example, relevance or recency.
AMPA receptor
an ionotropic glutamate receptor responsible for most excitatory fast neurotransmission in the central nervous system.
Catastrophic interference
neural networks store memories as patterns of activity, such that representations consist of a set of nodes and the weights of their connections. Because neural networks have a finite number

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