The 'state' in state-dependent learning is taken to imply the internal, physiological, and mental milieu, but its reinstatement often requires elements of the original external milieu ("context). The phenomenon is also termed 'dissociated learning'. The more general term 'state-dependent learning' is, however, to be preferred, as 'dissociation' connotes clinical meanings that are not intended.1 The terms 'state-dependent retrieval', 'state-dependent recall', and 'state-dependent memory' are also used, sometimes with an underlying assumption that the key to the understanding of the state dependency lies in a particular memory "phase but not in another. In practice, state dependency is frequently induced by drugs or intoxication: this special case of state-dependent learning is known as 'drug-dependent' or 'drug-dissociated' learning.
Over the years, the fascination with state-dependent learning has been shared by scientists and non-scientists alike. The latter, though, became intrigued much before the former. It was noted long ago that somnambulists (sleepwalkers, such as Lady Macbeth, Shakespeare 1606), could in their trance do things that are forgotten in their waking state but pursued or recalled in a subsequent episode of sleepwalking (Ellenberger 1970). The same phenomenon was later demonstrated with 'artificial somnambulism', i.e. hypnosis ('animal magnetism', ibid.), to which spontaneous somnambulists were found to be especially susceptible. This phenomenon of hypnosis-dependent learning was exploited not only by shamans, performing magicians, and occasionally crooks, but also by authors and screenwriters. In Dr Caligari, one of the greatest "classics of the German Expressionist cinema (Wiene et al. 1919), the notorious Caligari hypnotizes the somnambulist Cesare to commit murders while in a somnambulistic trance. Perhaps the most popular state-dependencies were, however, attributed to alcohol and narcotics. In describing a "system of "phrenology,
Combe (1830, cited in Siegel 1982) recounts the story of an Irish porter who used to forget, when sober, what he had done while drunk, and yet could recapture the memory once he again became indulged in drinking. This story was later echoed in the writings of several eminent physicians, including Winslow ('On obscure diseases of the brain', 1860, cited in Eich and Birbaum 1982) and Ribot (1882, "amnesia). Additional cases of alcohol-dependent recall also found their way into fictional literature (Dickens, cited in Siegel 1982) and the cinema (Chaplin, cited in Bower 1981).
In Victorian England opium was occasionally used as freely as alcohol (Berridge 1977), and hence opiate-dependent recall became known as well. Opiate-dependent recall was the clue to a contemporary best-seller, The moonstone (Collins 1868). In this detective story, a young gentleman, Blake, worried about the safety of his fiancé who possesses a valuable diamond, hides the diamond while he is under the effect of an opium tincture (laudanum). He has no memory of the event, till a physician's assistant advises him to recreate the drugged state. The author, Wilkie Collins, wrote from first-hand knowledge: he was himself an avid opium user, and the rumour went that he was unable to remember where in the novel he concluded the previous writing session unless he became drugged again (Siegel 1982).
Controlled scientific experimentation on state dependency has started with Girden and Culler (1937). They paralysed a dog with curare,2 and then trained it in a "classical conditioning protocol to associate the sound of a bell with a shock-induced leg flexion. After training, the conditioned response was displayed in the presence of curare, vanished on return to the normal nonparalysed state and reappeared with re-administration of the poison. Conversely, conditioning in the absence of curare was undetected in the presence of the poison. Drug-dissociation learning was subsequently demonstrated by many groups of investigators using a variety of drugs and species (e.g. Overton 1964; Bruins Slot and Colpaert 1999; smoking can do the trick as well, Peters and McGee 1982). In many studies of drug-dissociated learning, rather large doses of drugs are needed, that also affect acquisition. Even the anecdotal alcohol-dependent memory in Homo sapiens was respectively verified in the laboratory using a group of joyous volunteers (Goodwin et al. 1969). Recall of single experiences displayed a greater sensitivity to drug dependency than "recognition. This latter observation was later found to "generalize to other situations of state-dependent learning.
Many drugs that can produce drug-dissociated learning affect the mood. It is therefore only natural to ask whether affective states per se could induce state-dependent learning. The answer is probably yes (Blaney 1986). It was reported that depressed patients, or normal subjects hypnotized to sadness for the sake of science, display a 'mnemonic "bias' toward the recall of unpleasant information (Lloyd and Lishman 1975; Bower 1981; Clark and Teasdale 1982). Similar results have been reported for other emotional disorders (e.g. panic disorder favours panic-related words; Becker et al. 1994). The question is whether this is state dependency bona fide, i.e. whether the altered mood induces mood dependency in the retrievability of items in memory irrespective of the affective valence of these items. An alternative possibility is that an altered mood at the time of training or testing promotes the learning or recall, respectively, of events or facts that agree with that particular mood. In such a case concordance between mood at learning and mood at retrieval is not obligatory, i.e. it is not true state-dependent learning. The phenomenon in which material is more likely to be attended, learnt, or recalled if it is consistent with the subject's prevailing mood, is termed 'mood congruen-cy' or 'mood congruity' (Bower 1981; Blaney 1986). Overall, the jury is yet out on whether the available data on mood-associated recall favour state dependency, mood congruency, or both (Blaney 1986; Pearce et al. 1990; Power and Dalgleish 1997). Regardless of the exact mechanisms, if depressed patients display a bias for depressing memories, they could be expected to become immersed in depressing thoughts and expectations, which in turn will augment and perpetuate the depression. This is a vicious cycle that could contribute to chronic depression (Clark and Teasdale 1982; Ingram 1984; for a critical review of similar ideas and 'cognitive theories' of depression in general, see Haaga et al. 1991).
Recently, neuronal, "reduced analogues of state-dependent "plasticity and learning have been reported (Worgotter et al. 1998; Shulz et al. 2000). This, together with the analysis of "neurotransmitter and "receptor mechanisms in state dependency (Bruins Slot and Colpaert 1999), paves the way to further understanding of the molecular, cellular, and circuit mechanisms that implement state dependency at the behavioural "level. In the meantime, a practical caveat: the potential contribution of state-dependent learning to the outcome of behavioural experiments should not be ignored. A "control for state-dependent learning is highly advisable in every study in which drugs or conditional gene knockouts ("neurogenetics) are used to manipulate only the acquisition or retrieval "phase but not both. Furthermore, state dependency must be kept in mind whenever "developmental, hormonal, circadian rhythm, or "nutritional states are not kept under control throughout the experiment. All this, because memory traces may trick us and disappear, only to resurface later, when the original state experienced by the subject in acquisition is reinstated.
Selected associations: Context, Percept, Retrieval, Transfer
1in psychiatry, 'dissociation implies disruption of integrated functions of personality (DSM-iV 1994).
2A blocker of the 'acetylcholine *receptor that Native American warriors used to smear on their arrowheads.
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HYPNOTISM is by no means a new art. True, it has been developed into a science in comparatively recent years. But the principles of thought control have been used for thousands of years in India, ancient Egypt, among the Persians, Chinese and in many other ancient lands. Miracles of healing by the spoken word and laying on of hands are recorded in many early writings.