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MEMORY AND DRUGS: STATE DEPENDENT LEARNING The term state dependent learning (SDL) refers to the fact that memories acquired while a person is drugged may be forgotten when the drug wears off and not remembered until the person again takes the drug. Conversely, material learned in the undrugged state may be forgotten when a drug is taken; and material learned under one drug may be forgotten when another drug is used. SDL is sometimes called drug dissociation of learning, referring to the fact that material learned while drugged is dissociated from normal consciousness and not able to be retrieved.

Throughout the nineteenth century, there was a high level of public interest in multiple personality, fugue states, and other types of episodic amnesia; SDL was first reported in 1835 by George Combe, an English phrenologist, who viewed it as an analogous phenomenon, perhaps based on similar properties of the brain. SDL became an accepted property of mind during the latter half of the nineteenth century, and was a central theme in the plot of The Moonstone, (1868), a well-known mystery novel written by Wilkie Collins. Then, at the beginning of the twentieth century, interest in these dissociative phenomena waned and was replaced by an interest in the amnesias caused by repression, which Freud described. SDL was essentially forgotten.

SDL was rediscovered in the 1960s, this time in experiments using animals, and since then has been a popular topic of research and clinical speculation. Two types of mechanisms are postulated as possibly producing SDL. According to one theory, drugs produce sensory stimuli, subjective sensations— and one's ability to retrieve memories is aided by reinstatement of the stimuli that were present when learning occurred. A second theory suggests that some other property of brain results in memories being most easily retrieved when the conditions of brain excitability that were present during learning are reestablished. Sensory stimuli are not involved in producing SDL, according to this second theory. Thus far it has not been possible to confirm either of these proposed mechanisms experimentally, although the sensory model is more widely accepted.

SDL is produced only by drugs that act on the brain. There are marked differences in the strength of the SDL effects produced by the different centrally acting drugs. For example, BARBITURATES and ALCOHOL produce strong SDL effects, whereas chlorpromazine (Thorazine) produces almost no such effects. SDL is more likely to occur with high doses of drugs, and research on SDL has been severely hampered by the fact that these doses also produce other effects on memorization and retrieval that are difficult to distinguish from SDL effects. Some research suggests that the relative ability of different drugs to produce SDL may differ depending on the type of task that is employed, but this conclusion is not yet well substantiated.

Many consider SDL to be closely related to drug discriminations, believing that the discriminative control exercised by drug conditions is produced by the same drug effects that produce SDL amnesias at higher doses.

After SDL was rediscovered in the 1960s, clinicians feared that the lessons of psychotherapy carried out while a patient was drugged might be forgotten when drug treatment was discontinued. Subsequent studies showed that strong SDL effects typically did not occur except at doses higher than those normally employed during chronic treatment with psychotropic drugs. Some evidence, however, suggests that the stimulant drugs used to treat hyperactive children may produce SDL in those children. There is increasing evidence that some types of learning may take place under general anesthesia, although patients report they remember nothing after the anesthesia wears off. A considerable amount of research is currently focused on the possibility that SDL may block explicit recall of learning under general anesthesia, even though such learning occurs.

Many centrally acting drugs alter moods. A currently active area of research deals with the possibility that emotions act as memory cues and that memories learned in one emotional state may be recalled best when that emotion reoccurs; they may be recalled less easily at other times. Finally, there has been a dramatic increase in the number of reported cases of multiple personality disorder during the past decade. One of the theories used to explain this disorder holds that the process underlying it is similar, at a mechanistic level, to that which produces drug-induced SDL.

(SEE ALSO: Memory, Effects of Drugs on; Research; Animal Model)

BIBLIOGRAPHY

Collins, W. (1868/1981). The moonstone. New York:

Penguin Books. COMBE, G. (1830). A system of phrenology, 3rd ed. Edinburgh: John Anderson. OVERTON, D. A. (1991). Historical context of state dependent learning and discriminative drug effects. Behavioral Pharmacology, 2, 253-264. OVERTON, D. A. (1984). State dependent learning and drug discriminations. In L. L. Iversen, S. D. Iversen,

& S. H. Snyder (Eds.), Handbook ofpsychopharma-cology, vol. 18. New York: Plenum.

Overton, D. A. (1964). State-dependent or "dissociated" learning produced with pentobarbital. Journal of Comparative and Physiological Psychology, 57, 3-12.

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