Two component model of memory

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Outline of the model

The two component model of long-term memory asserts that two independent variables are sufficient to describe the status of unitary memory in a healthy human brain:

  • stability (S) - this variable determines how long a memory can last if undisturbed and if not retrieved
  • retrievability (R) - this variable determines the probability of retrieving a memory at a given time

Biological function

We believe the model is an expression of an evolutionary need to prevent catastrophic interference in neocortical networks. Stability of memory will reduce changes to synaptic weights and prevent forgetting of memories marked as highly useful. On the other hand, spacing effect protects memories from achieving high stability on frequent exposure (see: Structural and molecular mechanisms of the spacing effect).

Two component model of memory resolves the plasticity-stability dilemma


In literature on memory research, an ill-defined term of memory strength will often stand for either stability (S) or retrievability (R). This results in a great deal of confusion that slows down the progress of memory research. In 1990 and 1995, we have proven theoretically that the two variables: S and R are sufficient to describe the status of memory in spaced repetition.

Independent research by Bjork, formulated as New Theory of Disuse (1992), uses analogous terms of storage strength and retrieval strength. See also: Jost's Law.

For a historic take on the development of the model see: Two components of memory


The model can be used in the analysis of molecular and structural properties of long-term memory.

The ultimate practical proof for the correctness of the two component model comes in the implementation and practical outcomes of Algorithm SM-17 in SuperMemo. The model and the algorithm make it possible to quantitatively study memory. For example see: How much knowledge can human brain hold?

The model provides evidence in support of the grandmother cell theory. See: The truth about grandmother cells

This glossary entry is used to explain "History of spaced repetition" by Piotr Wozniak (June 2018)

SuperMemo: Changes in two variables of long-term memory: retrievability and stability
SuperMemo: Changes in two variables of long-term memory: retrievability and stability

Figure: Changes in memory status over time for an exemplary piece of knowledge. The horizontal axis represents time spanning the entire repetition history. The top panel shows retrievability (tenth power, R10, for easier analysis). Retrievability grid in gray is labelled by R=99%, R=98%, etc. The middle panel displays optimum intervals in navy. Repetition dates are marked by blue vertical lines and labelled in aqua. The end of the optimum interval where R crosses 90% line is marked by red vertical lines (only if intervals are longer than optimum intervals). The bottom panel visualizes stability (presented as ln(S)/ln(days) for easier analysis). The graph shows that retrievability drops fast (exponentially) after early repetitions when stability is low, however, it only drops from 100% to 94% in long 10 years after the 7th review. All values are derived from an actual repetition history and the three component model of memory.