The role of discriminative stimuli in modulating drug action.

Behavior reinforced in the presence of a stimulus comes under the control of the stimulus. A drug can then modify that control and, therefore, modify the behavior itself. Studies over the past 2 decades have shown that the nature of the controlling (or discriminative) stimulus can govern the degree to which drugs change performance. These experiments usually have compared behavior on various schedules of reinforcement with and without added discriminative stimuli. For instance, pigeons that had been trained on a fixed-interval schedule showed great changes in response distribution after amphetamine and scopolamine. The same birds, when performing on a fixed-interval schedule to which time-correlated discriminative stimuli had been added, showed smaller changes in response distribution. Other pigeons were trained to make a minimum number of consecutive responses on one key before a peck on a second key would be reinforced; beta-amphetamine and scopolamine led to pronounced increases in premature switching. Adding a discriminative stimulus when the response requirement was fulfilled increased the likelihood that a switch would occur only after the appropriate number of pecks had been emitted. It also attenuated the effects of the drugs. The presence of discriminative stimuli did not make as large a difference in performance in either of these experiments when chlorpromazine and promazine were studied. In general, work with other schedules of reinforcement supports the conclusion that behavior under stron external stimulus control is less apt to be readily affected by many drugs. Addition of the discriminative stimulus can also "improve" the behavior of pigeons that have been given enough methylmercury to increase greatly the variability of their performance.