Under certain circumstances, a response that has previously been acquired in the presence of a given discriminative stimulus may not currently be evoked by the presence of that stimulus. There are many factors that temporarily weaken the effectiveness of a given stimulus in increasing the probability of a given response. These include the passage of time, other interfering stimuli, a stimulus that has only a few features in common with the stimuli under which the response was acquired, illness, and the effects of drugs. Even though the response is not forthcoming under ordinary circumstances, this does not mean that its probability of occurrence has not been increased to some extent.
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