Memory consolidation

The broad definition of memory consolidation is the process by which recent memories are crystallised into long-term memory. The word "consolidation" is used to refer to different levels:


 * 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).

Reconsolidation
There is evidence in laboratory animals that recall puts memories into an unstable, labile state and that, after recall, the memory must be re-consolidated or it will be forgotten. 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 re-consolidation occurs upon review or repetition of the learned material.

Moreover, memory reconsolidation also may be disrupted by blocking NMDA receptors.

Research papers of interest
PMID: 15450162
 * 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.
 * Evidence that learning-related neuronal activity in the cerebral cortex rises more slowly than in the basal ganglia, suggesting the basal ganglia may "train" this segment of cortex.
 * 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.