Memory consolidation

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Memory consolidation is a set of processes that convert short-term memories into coherent and stable long-term memories. Some of those processes occur in sleep.

In the context of SuperMemo, consolidation is defined as the impact of review in SuperMemo on the level of recall at next review. For example, on day Dx, the level of recall might be 92%, but the level of consolidation will be known only when all items reviewed on day Dx are reviewed again in the future (which may be decades later). For atomic memories, memory consolidation occurs primarily via the increase in memory stability.

Research shows that recall and consolidation are in linear relationship. This means that days with good recall bode well for future consolidation. In simple terms, on good days, the brain is good at both (1) recall and at (2) re-consolidation of memories.

In memory research, it is important to distinguish between synaptic consolidation and systems consolidation. In terms of the Neurostatistical Model of Memory, this distinction is very sharp. Synaptic consolidation is outwardly expressed as memory stabilization, while a set of processes involved in systemic consolidation can be referred to as optimization of memory (of which a great deal occurs in sleep).

This glossary entry is used to explain texts in SuperMemo Guru series on memory, learning, creativity, and problem solving

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Good memory recall improves memory consolidation
Good memory recall improves memory consolidation

Figure: Memory consolidation is better on days characterized by a higher level of recall. The relationship between consolidation and recall is nearly linear. The graph was plotted using over 1.1 million repetitions in SuperMemo. Over 600,000 of those repetitions contributed to consolidation data. Consolidation levels with fewer than 3,000 data points have been omitted from the graph. The Deviation parameter says how well the linear fit matches the data (the less the deviation, the better the fit). The deviation is computed as a square root of the average of squared differences between the approximation and the data

Uncertain course of stabilization in complex memories
Uncertain course of stabilization in complex memories

Figure: Uncertain course of the stabilization of complex memories. The picture shows a hypothetical course of stabilization, forgetting, generalization, and interference on the example of a single dendritic input pattern of a single concept cell. The neuron, dendrites and dendritic filipodia are shown in orange. The picture does not show the conversion of filopodia into dendritic spines whose morphology changes over time with stabilization. The squares represent synapses involved in the recognition of the input pattern. Each square shows the status of the synapse in terms of the two component model of long-term memory. The intensity of red represents retrievability. The size of the blue area represents stability. After memorizing a complex memory pattern, the concept cell is able to recognize the pattern upon receiving a summation of signals from the red squares representing a new memory of high retrievability and very low stability. Each time the cell is re-activated, active inputs will undergo stabilization, which is represented by the increase in the blue area in the input square. Each time a signal does not arrive at an input while the concept cell is active, its stability will drop (generalization). Each time a source axon is active and the target neuron fails to fire, the stability will drop as well (competitive interference). Due to the uneven input of signal patterns to the concept cell, some synapses will be stabilized, while others will be lost. Forgetting occurs when a synapse loses its stability and its retrievability and when the relevant dendritic spine is retracted. Generalization occurs when the same concept cell can be re-activated using a smaller, but a more stable input pattern. Retroactive interference occurs when a new input pattern contributes to forgetting some of the redundant inputs necessary for the recognition of the old input pattern. Stabilization of the old patterns results in the reduced mobility of filopodia, which prevents the takeover of a concept by new patterns (proactive interference). At the every end of the process, a stable and a well-generalized input pattern is necessary and sufficient to activate the concept cell. The same cell can respond to different patterns as long as they are consistently stabilized. In spaced repetition, poor choice of knowledge representation will lead to poor reproducibility of the activation pattern, unequal stabilization of synapses, and forgetting. Forgetting of an item will occur when the input pattern is unable to activate sufficiently many synapses and thus unable to reactivate the concept cell. At repetition, depending on the context and the train of thought, an item may be retrieved or forgotten. The outcome of the repetition is uncertain