The ability of placebo drug capsules to serve as conditioned reinforcers as a function of being paired with differential rates of point reinforcement was evaluated. Normal volunteers were administered two differently colored capsules orally in separate sessions. Although different in color, both capsules were placebos. The volunteers were told that these capsules might contain a stimulant, sedatives, or placebo. During each session, the volunteers participated in performance tasks. Subjective effects and physiological measures were also obtained. The tasks, which were difficult and ambiguous, were programmed so that following the administration of one color capsule, the frequency of reinforcement was markedly greater than following the administration of the capsule of the other color. Volunteers were told that the difference in point earnings was related to the accuracy of their performance. During the first experiment, participants were exposed two times each to the two reinforcement conditions. During choice sessions, no performance tasks were done but participants were offered the opportunity to choose which capsule they preferred to self-administer. During the exposure sessions, there were large differences in the mood effects associated with the two capsules and the reinforcement conditions. During choice sessions, most participants chose the capsule associated with the high frequency of reinforcement. A second experiment was designed to determine whether the differential mood effects associated with the differential reinforcement conditions could be conditioned to the color of the placebo capsule associated with them. Although similar differences in mood were seen during sessions when performance testing was done, there were no differential mood effects observed when the two different colored capsules were administered in the absence of performance testing. These results indicate that drugs may function as conditioned reinforcers and thus influence the probability of drug-taking behavior.
