Dreams

Memory Consolidation and Reinforcement

Regarding this processing, during nonrapid eye movement (NREM) sleep, recently acquired short-term memories stored temporarily in the hippocampus become converted into long-term memories stored in the neocortex by a process known as hippocampal replay. In addition, enormous numbers of already-stored fragments of memories in the neocortex become reinforced (strengthened). During rapid eye movement (REM) sleep, many already-stored, long-term memories in the neocortex also are reinforced to maintain their authenticity.

Because dreams usually are highly visual, the storage process for visual memories is used here for illustration, though essentially the same principles apply generally. Storage of memories in both the hippocampus and neocortex is sparse and distributed, sparse in the sense that only a fragment of the memory is represented at any one of the distributed physical locations. In the visual neocortex, sets of neurons having different response properties (fragments) for colors, textures, distances, orientation, positions, and so forth, are clustered together at the various distributed locations.

As an example of memory consolidation by hippocampal replay, consider the process for a new declarative memory, that is, for the conscious recollection or explicit remembering of a new scene or event. During replay, fragments of the memory, already sparse and distributed and stored in the hippocampus for the short term, become similarly established in the neocortex for the long term, by a repetitive interactive process. Such replay in humans might continue for as long as 3 years.

Spontaneous reinforcement of memories stored in neocortical circuits during sleep involves electrical activations by both fast and slow brain waves. Reinforcement often is accompanied by an “unconscious” awareness of the corresponding memories in the forms of static dreams, that is, isolated thoughts or perceptions. Narrative dreams are assembled from these thoughts or perceptions but, on any given night, only from a small fraction of them.

It is an intrinsic property of synapses of memory circuits that they need to be reinforced periodically. Otherwise the synapses weaken and the encoded memories deteriorate, in days, weeks, or little more than a month. Synaptic strengths (weights) persist for only limited periods, primarily because the macromolecules that are essential for synaptic function break down continuously (molecular turnover). Because sufficient numbers of these molecules are needed to preserve the specific synaptic strengths that encode given memories, if “lost” molecules were not replaced periodically, these strengths would gradually decline and the memories would deteriorate.

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