Abstract

The effects of methamphetamine, phencyclidine and Δ⁹-tetrahydrocannabinol on responding under differential reinforcement of low rate schedules (DRL schedules) were studied under three different DRL time requirements. Under the DRL schedules studied, rats were required to space responses at least a minimum, but not more than a maximum, time interval apart. The time intervals between responses (interresponse times, or IRTs), when plotted as a frequency distribution, were usually a normal distribution with the peak at or near the minimum IRT required for delivery of the reinforcer. Methamphetamine flattened the IRT distribution and increased the frequency of long pauses under the DRL 1–1.3 sec schedule, but shifted the IRT distribution toward shorter IRTs under the DRL 4–5.2 and 10–13 sec schedules. Under the DRL 1–1.3 sec schedule, phencyclidine also increased long pauses. Under the DRL 4–5.2 sec and 10–13 sec schedules, phencyclidine produced dual effects on the IRT relative frequency distributions producing increases in the proportion of short IRTs similar to methamphetamine at low doses, but higher doses increased long pauses as well. Δ⁹-Tetrahydrocannabinol had little effect on responding under the DRL 1–1.3 sec and DRL 4–5.2 sec schedules, but it greatly increased the relative frequency of short IRTs under the DRL 10–13 sec schedule. Thus the effects of drugs on responding under these DRL schedules depended on the drug, the dose and the time requirements of the schedule, which suggests that a simple description of the effects of drugs on timing behavior or time perception is inadequate.