Explanations of Forgetting in Short Term Memory

March 25, 2019

Golden Papers

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The Decay Theory of Forgetting The decay theory of forgetting in STM states that forgetting is due to disruption of the active trace. The active trace is the engram that is formed when learning, which is very delicate. With learning the engram grows stronger until a permanent engram is formed, which is called a structural trace. One limitation to the theory is that it attempts to explain why forgetting increases with time. But this has been shown may not be true.

By a study of Jenkins and Dallenbach, 1924, forgetting was increased when participants were awake, more than for the participants who went to sleep right afterwards. Another limitation that was proposed by Solso 1995, is that there is no evidence that the major cause of forgetting from LTM is neurological decay. The Peterson and Peterson (1959) experiment has provided evidence of the role of decay in STM forgetting. If decay occurred, recall of information would be poorer by time.

They showed rapid forgetting of trigrams and it was concluded that the forgetting was caused by decay. The results of the experiments show good evidence for the theory. Hebb (1949) argued that when learning, an engram will be formed, which is very delicate and vulnerable to disruption. But with learning, the engram grows stronger until a permanent engram is formed, which is called the structural trace) through changes in neurons. The part where the engram is vulnerable to disruption during the active trace stage, could account for the decay theory.

One limitation to this theory is that there is no direct connection between the active trace and decay. It is not explained very well in the description. The Displacement Theory in Forgetting The displacement theory in forgetting states that when a system is ‘full’, in STM where there is limited capacity, old information may be displaced by new incoming information that pushes out the old information. Two limitations to this theory is that there is not a clear distinction between the effects of decay and displacement.

An experiment conducted by Shallice, the serial probe technique experiment, showed that decay must be a factor in forgetting since displacement theory should not have been affected by the test. The multi store model can be used to support the displacement theory in that STM is limited to 7 +/- 2 items, and that if information filled into these slots were pushed out without being rehearsed enough to allow it to go into LTM, the information would be lost. The two theories can support each other.

One study that can be used to support the displacement theory is the experiment conducted by Waugh and Norman (1965). They used the ‘serial probe task’, which involved participants presented with 16 digits at the rate of either one or 4 per second. One of the digits were then repeated, and participants were then asked to say the digit that had followed the probe. If the probe was in the beginning of the list of digits, the recall would be small, since the displacement theory states that new material pushes out old material.

If the probe was toward the end of the list, the recall would be easier since it had not yet been pushed out by new information, and would be available in STM. Since less time had elapsed between presentation of the digits and the probe in the four per second condition, there would have been less opportunity for the digits to have been decayed away. This makes it unclear if displacement is a process unaffected by decay. The Peterson and Peterson experiment on the duration of STM can be used as evidence for the displacement theory in that by removing the factor of rehearsal, the duration of memory could be studied directly.

I think the Peterson and Peterson experiment supports the decay theory better than the displacement theory. Ask for help. One criticism of this theory is that displacement is not directly and clearly supported by any experiments, if so, it is very vague. It is difficult to say which of the two theories are better in explaining forgetting in STM, partly because it is difficult to distinct the two from each other. It is likely that the two theories both contribute to forgetting in STM. There has been no studies so far that can draw a clear line between the two.