The effects of D-amphetamine on responding for candy and fruit drink using a fixed ratio and a progressive ratio schedule of reinforcer delivery.

The first purpose of this study was to compare the effects of D-amphetamine (AMPH) on operant responding reinforced under fixed ratio (FR) or progressive ratio (PR) schedules of reinforcement, testing the hypothesis that responding reinforced under a PR operant schedule would be disrupted by lower doses of AMPH than responding reinforced under a FR operant schedule. The second purpose of this study was to test the generalizability of the first hypothesis by comparing the effects of AMPH on responding reinforced by two different reinforcers under both FR and PR operant schedules. Rhesus monkeys had five to six candy and five to six fruit drink sessions per day, and could receive two reinforcers per session. Responding was initially reinforced under a PR procedure, such that the ratio size increased with each subsequent session. The parameters of the PR schedule were individually selected so that monkeys consumed a similar number of candy and fruit-drink reinforcers each day. The effects of oral AMPH (0.5, 0.75, 1.0 mg/kg) on responding were assessed. Responding was then stabilized using a FR schedule with parameters individually selected so that monkeys consumed a similar number of candy and fruit-drink reinforcers each day, and the effects of oral AMPH were again assessed. The PR breakpoint was significantly greater for candy than fruit-drink. AMPH produced dose-related decreases in both candy and fruit-drink intake, but each AMPH dose decreased the number of fruit-drink deliveries to a greater extent than the number of candy deliveries. The results failed to support the hypothesis that responding under PR schedules of reinforcement would be disrupted by lower doses of AMPH.