Theories of Forgetting

Explaining Memory Failure

Forgetting as described earlier Opens in new window is simply the loss of learned information over time. A number of theories have been put forward to explain why forgetting takes place.

Most (but not all) of these theories are designed to account for forgetting where long-term memories are available but not accessible. The main theories of forgetting are briefly summarized below.

1. Spontaneous decay

The spontaneous decay theory (Ebbinghaus, 1885) proposes that memories simply fade away with the passage of time. This theory seems to offer a simple explanation for the time-dependent memory loss seen in the classic Ebbinghaus forgetting curve (shown below).

Figure X-1 | Ebbinghaus' forgetting curve and review cycle. Source: ResearchGate Opens in new window

However, most other theories of forgetting can also explain this finding. Research over the last century has failed to provide any clear evidence for the occurrence of spontaneous decay in long-term memory Opens in new window, and indeed there is evidence that memories do not seem to decay during periods of low brain activity, for example when we are asleep.

Consequently many researchers have concluded that there is no clear evidence for the spontaneous decay theory (McGeoch, 1932; Brown & Lewandowsky, 2010).

However, decay theory cannot be totally ruled out as a possible cause of memory loss, and some recent studies have provided possible evidence for its existence (Altmann & Schun, 2012; Hardt et al., 2013).

2. Decay with disuse

Thorndyke (1914) put forward a rather different version of the decay theory, in which it is argued that memories will only decay if they remain unused for a long period of time.

A more recent version of this theory, known as the New Theory of Disuse (Bjork & Bjork, 1992), proposes that access to a memory trace is strengthened whenever it is retrieved. A memory that is never retrieved will therefore eventually become inaccessible. However, this may be a result of an inhibitory process removing unused and unwanted memories, rather than being the result of spontaneous decay.

The New Theory of Disuse receives support from the finding that access to a memory becomes stronger when it is frequently retrieved (the testing effect).

3. Interference

The interference theory (Ebbinghaus, 1885) proposes that access to a memory can be impeded or blocked by the input or presence of other items in the memory store. There is a considerable amount of evidence confirming that interference Opens in new window can impede access to a memory (Ebbinghaus, 1885; Underwood & Postman, 1960; Wixted, 2010).

Interference can arise from previously learned items (proactive interference Opens in new window) or from items learned subsequent to input (retroactive interference Opens in new window).

One possible explanation of interference is based on the principle of response competition, where a number of different memories are competing for retrieval Opens in new window, thus raising the possibility that the wrong one may be retrieved.

Whilst earlier studies mostly focused on the interfering effect caused by the input of other new material, interference has also been found to arise from the retrieval of rival items. This is known as output interference, and it has been shown to have a significant effect on memory performance (Tulving & Arbuckle, 1963; Anderson, 2003).

4. Ineffective retrieval cues

Retrieval cues can help to activate a memory, but for a cue to be effective it must match up in some way with the content of the memory trace which was originally placed in storage.

In other words there must be some similarity or feature overlap between the retrieval cue and the stored memory trace, a hypothesis known as the Encoding Specificity Principle (Tulving & Thompson, 1973).

This theory receives support from many different findings, notably the occurrence of Transfer-Appropriate Processing (Morris et al., 1977), which is the finding that memories are more retrievable when the type of retrieval processing at the output stage resembles the processing that was carried out at the input stage.

The Encoding Specificity Principle (ESP) also provides a possible explanation for the finding that retrieval is improved when the initial learning context and surrounding are reinstated at the retrieval stage (Greenspoon & Ranyard, 1957; Godden & Baddeley, 1975), a phenomenon known as Context-Dependent Memory.

5. Cue overload

Another kind of retrieval-failure theory proposes that memory failure may be caused by cue overload (Earhard, 1967; Wixted, 2010), whereby a retrieval cue becomes associated with a number of different memory traces, thereby reducing the chance of retrieving the correct one.

Cue overload theory resembles some aspects of interference theory, as memory failure is assumed to be caused by the presence of competing memory traces.

6. Repression

Freud (1957) suggested that memories we find unpleasant or distressing may sometimes be repressed, which means that they are forced into the unconscious part of the mind so that we are no longer able to retrieve them consciously.

However, Freud believed that repressed memories could still exert an influence over our conscious thoughts and behavior, and in fact the uncovering and release of such repressed memories was an important part of Freud’s psychoanalytic therapy.

Repression Opens in new window is an example of motivated forgetting, since it involves the purposeful suppression of unwanted memories. However, the existence of repression as a cause of forgetting remains controversial.

7. Consolidation

Consolidation is the process by which temporarily memories held in the conscious working memory are converted into more permanent memory traces stored in the long-term storage memory Opens in new window. In typical cases of organic amnesia Opens in new window there is severe impairment of the storage memory but a relatively intact working memory Opens in new window, which suggests that in such cases amnesia is caused by a failure of the consolidation process (Milner, 1966).

Consolidation Opens in new window failure occurs in normal non-amnesic individuals too, especially if they are distracted or overloaded by other perceptual input, or simply failing to pay full attention to the target input.

It could be argued that the failure to consolidate a memory is a disorder of learning and encoding rather than memory failure as such.

However, there is some evidence for the occurrence of a more gradual form of consolidation which may take months or even years to be completed (Squire, 1992b; Steinvorth et al., 2005), which can affect the strength of a memory long after its initial acquisition. In practice, memory loss due to consolidation failure is most evident in amnesic individuals, but it may possibly occur in non-amnesic individuals too.

8. Reconsolidation

Some recent studies have shown that the retrieval of an item from memory may bring about further consolidation of that memory (Nader et al., 2000). This is known as reconsolidation, and it causes changes in the memory trace which can make its accessibility either stronger or weaker.

Most studies of reconsolidation involve the use of psychoactive drugs to alter the trace during retrieval, but it has been found that psychological interventions such as emotional arousal can also produce reconsolidation effects (Finn & Roediger, 2011).

These reconsolidation effects are therefore likely to occur naturally in everyday life, and they may be involved in some types of forgetting.

9. Inhibition

There is some evidence that inhibitory systems may be at work in the brain, whose purpose is to remove or weaken unwanted memories. Indeed such an inhibitory system could be the mechanism underlying the kind of adaptive forgetting described here Opens in new window.

There is some evidence for the existence of such inhibitory systems. For example, it has been shown that the retrieval of one item from memory can make other similar items harder to retrieve (Anderson et al., 1994), a phenomenon known as Retrieval-Induced Forgetting (RIF).

Anderson (2003) argues that the act of retrieving an item activates an inhibitory mechanism whose purpose is to suppress the retrieval of rival items, in order to facilitate selective retrieval.

However, not all researchers accept this explanation of RIF, and some have proposed alternative explanations which do not involve an inhibitory mechanism (Jonker et al., 2015).

At the present time inhibitory theories of memory remain hypothetical, but they do offer a possible explanation for some aspects of memory function.

The theories of forgetting discussed above are not totally separate and independent of one another, and in fact some of them may overlap considerably. For example, it is possible that memory inhibition could be the mechanism underlying the occurrence of decay with disuse, or possible interference.