Regular coffee drinkers show tolerance to the salivation-inducing effects of caffeine. We present evidence indicating that this tolerance results from a conditioned inhibition of salivation, with coffee as the conditioned stimulus. The tolerance disappears when caffeine is presented in an unfamiliar vehicle, and inhibition of salivation occurs when coffee drinkers drink decaffeinated coffee. These two findings are predictions of a conditioned opponent view, which holds that stimuli associated with the administration of caffeine induce physiological conditioned responses that oppose the basic effects of the drug. In contrast with salivation, the alerting effects of caffeine show little tolerance, and no evidence for conditioned opponent processes. Thus, different effects of the same drug can perhaps display totally different conditioning properties.
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