Conditioned reinforcing effects of capsules associated with high versus low monetary payoff.

The ability of a placebo drug capsule to serve as a conditioned reinforcer as a function of being paired with money reinforcement was evaluated. Volunteers were administered two differently colored capsules that presumably contained two different drugs. Although the volunteers were told they might contain a stimulant, sedative, or placebo, both capsules contained only a placebo. During sessions, volunteers participated in performance tasks. The tasks were programmed so that following one capsule, the amount of money obtained contingent upon responding was greater (high frequency of reinforcement) than following the other capsule (low frequency of reinforcement). During experiment 1, participants were exposed twice each to the two reinforcement conditions (sampling). During these choice sessions, 9 of 12 participants chose the capsule associated with the high frequency of reinforcement 2 or 3 times. Experiment 2 was designed to explore further whether the differential mood effects observed during sampling sessions could be conditioned. Although this could not be demonstrated, the self-administration results demonstrating the control of choice behavior even in the absence of pharmacological effects suggest that drugs may function as conditioned reinforcers. This finding has implications for broadening our understanding of the determinants of initiation and continued drug use.