The deterioration of correspondence between the memory retrieved and the memory acquired

Herman Ebbinghaus, the progenitor of quantitative experimental psychology (Gorfein and Hoffman 1987), remarked that 'all sorts of ideas, if left to themselves, are gradually forgotten' (Ebbinghaus 1885). Whether this statement is acceptable by laypersons and scientists alike, depends on what is meant by 'forgotten'. If'forgot-ten' means 'erased from memory', then many, including professional psychologists, tend to believe that everything we learn is never erased (Loftus and Loftus 1980). In contrast, others, starting with Plato (Theaetetus 191), trust that memory traces can indeed be obliterated ('drowned in the waters of Lethe', Galton 1879).1 As it is much more difficult, and frequently impossible, to distinguish an obliterated memory from a nonretrievable one, the use of'retrieval' in the definition of'forgetting' (definition 2 above) is therefore a safer bet.

Forgetting occurs in all species (elephants included) and "memory "systems ("taxonomy), but its magnitude and kinetics depend on the type of memory, on its use, and on the conditions under which it is tapped (Baddeley 1997). The variables involved are not straightforward. People display surprising forgetfulness even for personal events that could be expected to rank as important indeed to the "subject, ranging from salient changes in personal status (Jenkins et al. 1979) to routine dietary behaviour (Smith 1991). And when a frequently retrieved memory does seem 'unforgetful'— is it the original "engram that is reinforced with repeated use, or, alternatively, a changing engram that is reconstructed and then "consolidated each time anew ("flashbulb memory, "real-life memory)? In most memory "paradigms one is able to come up with 'forgetting curves' (Ebbinghaus 1885), which show deterioration in "performance over time for almost every material learned, except that in some cases the time-scale is minutes or hours (e.g. nonsense syllables, ibid), whereas in others it is years to decades (e.g. a foreign language or autobiographical episodes; Linton 1978; Bahrick 1984). A useful method to quantify forgetting, or actually [1-forgetting], in the laboratory is to measure 'saving'. This is the decrease in the amount of training needed on retraining on the original task (hence measuring 'the saving of work in the case of relearning'; Ebbinghaus 1885). "Real-life approaches to forgetting focus on the determination of the loss in details and veridicality of the memory (definition 3; Koriat etal. 2000; "false memory).

One could come up with multiple types of potential explanations for the various manifestations of forgetting. Four types of hypotheses concerning either true or apparent forgetting have specifically attracted the attention of experimentalists and theoreticians (Freud 1901, 1915; Pavlov 1927; McGeoch 1932; Underwood and Extrand 1966; Shiffrin and Atkinson 1969; Tulving 1983; Capaldi and Neath 1995).

1. Forgetting occurs because the biological substrate that encodes the engram disintegrates or decays with time. This decay may involve the entire representation or, more likely, fragments of it, up to a point where degradation ceases to be 'graceful' ("persistence) and becomes catastrophic.

2. Forgetting occurs because the learned information is processed in a way that erases part(s) of the engram. This could be the consequence of either passive or active processes. 'Passive' means that with mere usage, the fidelity of the information deteriorates, and the representation gets noisy and ultimately meaningless. 'Active' means that information is actively being pruned over time, or 'unlearned', to optimize storage "capacity, retrieval, or performance (Hopfield et al. 1983; McClelland et al. 1995; "consolidation).2

3. A related possibility is that forgetting, or at least apparent forgetting (see below), occurs because other information alters the engram or interferes with its expression. The interference may be 'proactive' (of earlier learning on later learning), or 'retroactive' (of later learning on the recall of previously learned information; see examples in "classical conditioning). A notable case is "experimental extinction, in which an acquired "association is inhibited by repetitive post-training presentation of the unrewarded stimulus. The original information could still exist, and in experimental extinction of associative conditioning indeed it does, but it becomes ineffective in controlling behaviour. As noted above, the interaction may also occur in

're"consolidation' of the memory immediately after its retrieval. A special type of interference is promoted in psychoanalytic theory. This is 'repression', a defence mechanism3 that attempts to turn anxiety-provoking memories non-retrievable (Freud 1915; Laplanche and Pontalis 1973; "infantile amnesia, "palimpsest).4 Some authors will argue that interference mechanisms are not bona fide forgetting, because the original trace is not really obliterated; yet, as already noted, unless one deals with a straightforward case of experimental extinction, practically, it is usually difficult to determine whether a forgotten memory is abolished or only repressed. 4. Forgetting is due to the lack of appropriate retrieval "cues or to the use of an inappropriate processing mode in retrieval ("transfer). This again is apparent forgetting; given the appropriate cues and processing, the memory will be actualized.

Although forgetting is frequently regarded as a nuisance, for example, when it involves tasks to be performed in daily life ("prospective memory), the real nuisance might be not to forget at all. 'Thus even a happy life is possible without remembrance, as the beast shows; but life in any true sense is absolutely impossible without forgetfulness' (Nitzsche, cited in Roth 1989). People who suffer from 'hyperamnesia', the antipode of "amnesia, are miserable and would rather forget [two "classic cases are Monsieur X of Guillons, cited in Roth (1989), and the mnemonist S of Luria (1969)]. Forgetting could permit us to increase the signal-to-noise ratio of our cognitive narratives, and "generalize about the world. Occasionally it could also facilitate the acquisition of new "skills, by weakening distractive, older ones. And, last but not least, forgetting bad memories could smoothen the roughness of life. We should not forget that.

Selected associations: Amnesia, Cue, Experimental extinction, Infantile amnesia, Retrieval

1Lethe was the mythological river in Hades, the Land of the Dead, whose water induced forgetfulness in those who drank it, Republic 621; see *lotus.

2An intriguing hypothesis is that 'unlearning', pruning, and reorganization of information occurs in dream sleep (Crick and Mitchison 1983; Sejnowski 1995). If this is the case, then we do not only forget dreams, we also dream to forget.

3In psychoanalysis, defence mechanisms are postulated unconscious mental operations that are aimed at the reduction of painful emotions, ideas, and drives (Vaillant 1992).

4For candidate repression processes in brain, see Anderson and Green (2001).

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