Possible amotivational effects following marijuana smoking under laboratory conditions.

Human participants earned money by responding on a progressive-ratio (PR) schedule (initial value $50) or received money without responding on a fixed-time (FT) schedule. During the session, participants could terminate the PR schedule and initiate an FT 200-s schedule. In Experiment 1, increases in monetary value produced increased number of responses, time spent, and money earned in the PR component. In Experiment 2, marijuana smoking produced potency-related reductions in the number of responses, time spent, and money earned in the PR component, effects that can be interpreted as amotivational. Increasing the monetary value of the reinforcer diminished the acute marijuana effects on PR responding, suggesting that marijuana exerted an effect primarily on reinforcers of a smaller magnitude.