Memory consolidation

Memory consolidation, broadly defined, is the process by which recent memories (short-term memories) are crystallised into long-term memory. The term "consolidation" is used to refer to different levels of organization:

1. Molecular consolidation: The molecular process by which long-term conductivity of synapses is affected. Memory consolidation occurs after training (e.g. an exposition to a stimulus-response pair). Consolidation increases in strength over time with repetition. Maximum consolidation with minimum time investment is achieved by means of spaced repetition. Molecular consolidation requires protein synthesis.
2. Network consolidation: Many researchers believe that episodic memories are initially stored in the hippocampus and are slowly moved (or 'consolidated') into the neocortex. This process of consolidation begins during wakefulness and may be enhanced during sleep. Originally it was thought this happens during dreaming (Marr, 1971). However, new research indicates that the NREM phase of sleep is associated with that process (Hobson, Stickgold, Buzsaki).

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Reconsolidation

Reconsolidation is the process of once again consolidating memories which are actively being recalled (such recalled memories have already been previously consolidated). It involves neural processes that are similar to those involved in the original consolidation. In laboratory animals, recall puts memories into an unstable labile state, and after recall, the memory must be reconsolidated or it will be forgotten. Memory reconsolidation occurs upon review or repetition of the learned material.

Both consolidation and reconsolidation can be disrupted by pharmacological agents (e.g. the protein synthesis inhibitor anisomycin) and both require the transcription factor CREB. Recent research suggests that BDNF is required for consolidation (but not reconsolidation) whereas the transcription factor and immediate early gene Zif268 is required for reconsolidation but not consolidation. Memory reconsolidation may also be disrupted by blocking NMDA receptors.

References

Research papers of interest

• Debiec J, LeDoux JE, Nader K. Cellular and Systems Reconsolidation in the Hippocampus. Neuron. 2002 Oct 24;36(3):527-38. PMID 12408854
• Lee J.L, Everitt BJ, Thomas KL. Independent Cellular Processes for Hippocampal Memory Consolidation and Reconsolidation". Science. 2004 May 7;304(5672):839-43. PMID 15073322
• Pasupathy A, Miller EK. Different time courses of learning-related activity in the prefrontal cortex and striatum. Nature. 2005 Feb 24;433(7028):873-6. PMID 15729344.
• D. Marr. Simple memory: a theory for archicortex. Philos Trans R Soc Lond B Biol Sci. 1971 Jul 1;262(841):23-81. PMID 4399412
• Dudai Y, Eisenberg M. Rites of passage of the engram: reconsolidation and the lingering consolidation hypothesis. Neuron. 2004 Sep 30;44(1):93-100. Review. PMID: 15450162