Memory optimization in sleep

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In the course of a night sleep, memories are weakened, strengthened, formed, deleted, or moved. Those processes serve to improve the quality of memories. Memories are supposed to get simpler, more durable (i.e. less prone to interference), apply to more situations (i.e. become more general), or be forgotten (if not too relevant). The process of learning, creativity and memory optimization is hooked up to the circadian cycle in natural creativity cycle.

In learning, memory coherence affects how memories are reinforced in sleep. Self-directed learning based on the learn drive is the most effective way of building coherent memories. At the same time, coercive education may destroy memories through interference, lead to island memories that are eliminated in sleep, or produce durable futile, parasitic or toxic memories.

Walker and Stickgold classify the outcomes of memory optimization in sleep into the following classes of new memories:

  • unitized memories: when memory associations are rewired into the neocortex (abandoning the mediation of the hippocampus)(memory consolidation)
  • assimilated memories: when new connections form with semantically related units (building memory coherence)
  • abstracted memories: when existing memories are used to apply to a larger class of problems (memory generalization)(see this Figure)

Abstraction may occur through forming new generalized concepts, or chiseling out the existing concepts through forgetting.

For more see: Neural optimization in sleep

This glossary entry is used to explain "Good sleep, good learning, good life" (2017) by Piotr Wozniak

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