Effects of morphine, d-amphetamine, and food deprivation on temporally organized behavior

Two experiments examined the effects of morphine and d-amphetamine on behavior that was temporally organized under several schedule arrangements. Two body weights were assessed for each experiment. Experiment 1 made use of fixed-interval (FI) schedules, the peak procedure, and a two-key (free-operant psychophysical) procedure. Indexes of timing performance under each of these schedules were assessed and compared. Timing indexes were decreased by both drugs in the two-key procedure, but were not reliably altered in the peak procedure. Both drugs produced clear rate-dependent effects on behavior in both procedures. In Experiment 2, FI performance was assessed both alone and with a concurrently available variable-interval (VI) schedule to determine whether drugs would cause a pigeon to leave a potential source of reinforcement early in order to respond to a future source of reinforcement. Although reductions in the index of curvature were obtained, there was no clear difference in FI performance based on whether or not a VI schedule was concurrently available. The results of Experiment 1 are inconsistent with the theory of a sped-up pacemaker, but are consistent with explanations involving a decrease in attention. The data also are consistent with the rate-dependency hypothesis, which states that rates of responding following drug administration tend to be determined in large part by rates of responding under baseline conditions.