Extinction is subject to savings, that is, influences from previous learning that persist and interfere with the permanent uncoupling of the associative link between the CS and US (Kehoe and Macrae, 1997). Despite many previous extinction trials, the CS may spontaneously recover and elicit the previously extinguished CR. In practice, the extinction process serves only to reduce the future occurrence of the CR, not eliminate it. The persistence of classically conditioned behavior is particularly evident and problematic in the case of fear conditioning, phobias, and aggression.
in addition to spontaneous recovery, the classical conditioning phenomenon known as disinhibition can interfere with extinction efforts. Disinhibition occurs when a startling or surprising event (a distraction sufficient to elicit an orienting response) is presented together with the extinguished CS. As a result of this arrangement, conditioned responding to the CS spontaneously reappears in spite of many previous extinction trials. Under natural conditions of dog training, these sorts of disinhibitory influences are impossible to avoid entirely, requiring instead that they be proofed against as part of the training process.
Several other sources of relapse have been identified in addition to spontaneous recovery and disinhibition (Bouton and Swartzen-truber, 1991): renewal, reinstatement, and reacquisition.
Renewal refers to the effect that a change of context has on the extinction of a CR. Contextual cues play a significant role in the learning and unlearning of behavior. In the typical renewal experiment, a conditioned fear response is first trained in one context and then extinguished in another. When the animal is placed back into the original context, the extinguished fear response is strongly renewed despite the intensive extinction efforts. Another variation on the renewal experiment involves testing the animal after extinction in a novel context. Fear is renewed in the novel context even though no previous conditioning has actually taken place there. These sorts of experiments indicate that the animal learns to express or inhibit fear depending on the degree of safety or danger associated with the situation—that is, extinction is to a significant extent context dependent.
Reinstatement of an extinguished CS occurs when the original US is presented in the absence of the CS. Later (after a day or more), the CS is tested and found to have recovered its ability to elicit the previously extinguished CR. The reinstatement effect plays an important role in the recovery of phobias. For example, a dog exposed to a particularly fearsome sample of thunder may recover previously extinguished fear-eliciting conditioned stimuli associated with thunder (overcast skies, barometric pressure changes, and distant lightning flashes).
Recovery effects are also evident during reacquisition training. When a previously extinguished CS is paired again with the US, the recovery of responding is much more rapid than when a NS is paired with the US. The degree of recovery during reacquisition training depends on the context and associated renewal effects. Contexts that have been associated with past aversive training tend to produce more rapid and robust reacquisition, whereas contexts that have been associated with safety tend to retard reacquisition.
These various recovery phenomena indicate that extinction does not entirely erase the associative link formed between the CS and US or degrade the encoded memory of previous emotional conditioning. instead of being conceived as a means for erasing past learning, extinction is best interpreted as an active learning process, incorporating and consolidating previously acquired associative information about the CS and US with new input from the environment. As such, extinction is dependent on both stimulus-specific associations between the CS and US, as well as contextual occasion setting cues. Learning about specific stimulus relationships and the contexts in which they occur provides the organism with a flexible and discriminating associative interface with biologically significant events.
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