Habituation and Extinction Models

Processes often cited as explanation for the effects of exposure are habituation and extinction.

Habituation refers to a decline in fear responses, particularly the physiological responses, over repeated exposures to fear-provoking stimuli. Habituation is regarded as an unlearned temporary reaction. Habitua-tion is supposed not to be affected by cognitions. The classical habituation theory predicts that habituation would not occur if (baseline) arousal were high. Then, arousal would further increase and lead to sensitization (i.e., increase in fear responding after repeated exposures to fear-provoking stimuli). However, the literature revealed that, for instance, during exposure to high fear-provoking stimuli (e.g., flooding therapy), a reduction instead of a further increase in psychophysi-ological and subjective anxiety could be expected. Recent habituation theories have been extended to accommodate these findings. These dual-process theories describe complex interactions between habituation and sensitization, in which habituation can eventually occur after exposure to high fear-provoking stimuli.

Several studies have provided supportive evidence for a role of habituation in exposure therapy. First, in several studies self-reported fear and physiological arousal show a declining trend across exposures consistent with habituation. Second, findings in specific pho-bics have revealed that physiological habituation is related to improvement, whereas nonhabituation is not. Third, variables independent of habituation (e.g., level of arousal, rate of stimulation, and regularity of presentation) seem to affect habituation and fear reduction in a similar manner.

A major difficulty in the habituation theories is the presumed short-term effect of habituation. The habitu-ation theory would predict a return of fear after a certain time interval without stimulus exposure. Studies, however, show that a considerable proportion of patients report to be free of anxiety during long-lasting periods of time. To account for the long-term effects of exposure, several theorists have made a distinction between short-term and long-term habituation. For example, it was suggested that long-term habituation depends on higher cognitive processes, whereas short-term habituation represents mostly autonomic damping. There are more limitations to the habituation theory. For example, the habituation model cannot adequately explain the persistence of some fearful responding despite repeated exposures.

Extinction refers to decrements in responding through repetition of unreinforced responding. Thus, extinction-based theories suggest that anxiety reduction results from repeated encounters with anxiety-provoking situations without aversive consequences. Unlike habituation, extinction is supposed to be affected by cognitions. In other words, it is assumed to be an active instead of a passive process. The extinction theory presupposes that phobic behavior has been learned and therefore can be unlearned. Several cognitive explanations have been put forward to explain what is learned during extinction of phobic behavior. For example, it is postulated that what is learned is disconfirmation of outcome expectations, or enhanced self-efficacy, or the fact that the arousal associated with exposure is not dangerous. These cognitive-oriented theories will be discussed in more detail in the following paragraphs.

Still another explanation is that the context determines which meaning a situation becomes. It has been suggested that the joint presence of the stimulus (e.g., palpitations) and the context (e.g., far from home) determine the meaning (e.g., heart attack). The latter theory is in line with evidence that return of fear may be minimized with prolonged exposure sessions to a variety of contexts.

Evidence that is in conflict with the extinction theory in explaining fear and anxiety reduction during exposure comes from "escape" studies. According to the extinction theory, patients have to remain in the fear-provoking situation until anxiety decreases substantially in order to learn that nothing bad happens. Several studies have demonstrated, however, that anxiety reduction can occur, even though patients were allowed to escape from the feared situation before reaching maximum anxiety. In these studies, it is unclear what is learned. The patients did not seem to have the time and chance to learn about the possible feared consequences associated with the feared situation.

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