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Memory decay refers to the loss of memory over time. There are generally three types of memory: Sensory memory, Short-term memory and Long-term memory.

Decay or inability to retrieve

Do we forget because the information is gone, or do we forget because we can't access information that is still there? It is difficult to distinguish the two. However, there is evidence that we retain more than we can retrieve.

Experiment: (Nelson 1971) - Learn paired associates (numbers to nouns). Tested 2 weeks later to see which were remembered. Then given new material to learn that had some of the forgotten numbers, both with and without their original nouns.

Results: Subjects relearned the original associations faster (in spite of the fact that they could not recall them). This suggests that some associative information was retained. One possible interpretation: strength of memories decay gradually. If these strengths fall below a certain threshold, we can't recall the information, but the remaining memory trace is still there to facilitate relearning.

Decay or interference

Is forgetting due to decay of unused information, or to interference of new information with old information? Different kinds of evidence are offered for each position.

A survey of forgetting research concluded that the rate at which we forget information usually conforms to a power law: we forget a lot at first, but over time the rate of forgetting diminishes.

Decrease in long-term potentiation follows a similar power law. These facts are interpreted by some as evidence for a physiologically determined decay rate.

Interference Experiments Typical Experiment (A-D C-D paradigm):

1. Subjects all learn A-B association (between items on list A and items on list B).

2. Experimental subjects learn A-D associations (which use the same stimuli items as the A-B associations), while control subjects learn C-D association.

3. Everyone is tested on A-B associations.

Typical Results: Experimental subjects take longer to learn their second set of associations than controls, and make more errors on the A-B test. Experimental subjects take longer to learn their second set of associations than controls, and make more errors on the A-B test. These results are interpreted as evidence that learning new associations to stimuli causes forgetting of old associations. However, interference does not happen with factual material when the additional facts are redundant with (e.g., causally related to) the original facts.

Fan Effect (a model) - Interference effects can be modeled as weakening of spreading activation over multiple links in a propositional network.

Stimulus activates concept nodes.- Fixed (limited) amount of activation spreads from activated nodes over associative links, divided equally between links. (Hence the more links, the less activation per link.) Activation converges at propositional nodes (candidate responses) until one emerges as the answer. Time to identify the response is inversely related to level of activation.

Decay or Interference? Some claim that interference can produce the appearance of decay although it appears, both mechanisms are involved in forgetting or memory loss.

Decay or Displacement

Decay: information that is not rehearsed disappears as time passes. Displacement: information being held in STM is pushed out by newly arriving information. Displacement is most likely to occur when the capacity limit of STM has been reached (about 7 units of information). The original version of the Atkinson-Shiffrin model emphasized decay as the main cause of forgetting in STM. Their later version emphasized displacement.

See also