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...