State-dependent memory

State-dependent memory or state-dependent learning is the phenomenon through which memory retrieval is most efficient when an individual is in the same state of consciousness as they were when the memory was formed. The term is often used to describe memory retrieval while in states of consciousness produced by psychoactive drugs – most commonly, alcohol, but has implications for mood or non-substance induced states of consciousness as well.

Unlike context-dependent memory, which involves an individual's external environment and conditions, state-dependent memory applies to the individual's internal conditions. For example, while context-dependent memory might refer to the idea that taking a test in the same room that an individual studied in will make it easier to retrieve those memories, state-dependent learning refers to the idea that if an individual always studied for a test while slightly caffeinated, it will most likely be easiest to recall what they studied during the test if they are at a similar level of caffeination.

In 1937, at the University of Illinois, Edward Girden and Elmer Culler conducted an experiment on conditioned responses in dogs under the influence of the drug curare. Their study taught dogs a conditioned muscular response – to draw their paw away from the ground when they heard a buzzer, which was often accompanied by a small electric shock that motivated this response. It was found that dogs that had been administered curare when they first learned the response would be less likely to remember to draw their paw away upon hearing the buzzer when the curare was no longer in their system. However, once curare was reintroduced to the dog, the desired behavioral response returned. This result indicated that there was a connection between the dogs' states of consciousness and their ability to recall the responses. Girden and Culler's research opened the door for further investigation of the influences of state of consciousness on an organism's ability to encode memory.

Following this discovery, other researchers looked into the effect of different states of being on the ability to learn and remember responses or information. In 1964, Donald Overton conducted a study as a direct response to Girden and Culler's 1937 experiment. The study tested the effects of sodium pentobarbital on rats' abilities to learn and remember certain taught responses. These rats were randomly assigned to one of two groups – substance administered or no substance administered (the control condition) – and then placed in a simple maze and taught to escape an electrical shock. Overton found that the rats that had been administered 25 mg of sodium pentobarbital could no longer remember the proper escape response when they were later placed in the maze without the drug. However, if these rats were administered sodium pentobarbital once again and placed in the maze, they recalled the escape response they had been taught. Similarly, when Overton taught a rat the escape response under the control condition (no sodium pentobarbital administered), it could not recall that behavior when it was administered the drug and asked to perform later on. Results strongly indicated that rats performed the learned response more efficiently when in the either sodium pentobarbital or control state that they were in when they first learned it. In regard to this idea the study specifically stated "a response learned under the influence of a particular drug will subsequently reoccur (with maximum strength) only when that drug condition is reinstated."

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