Abstract

The effects of methamphetamine, phencyclidine and Δ⁹-tetrahydrocannabinol on responding under temporal response differentiation schedules were studied under three different time requirements. Under the schedules studied, Sprague-Dawley rats were required to make a continuous response for at least a minimum time duration, but not more than a maximum. Base-line performance under a temporal differentiation schedule usually produces a normal frequency distribution of response durations with the peak at or near the minimum duration required for delivery of the reinforcer. These frequencies were summed to calculate cumulative frequencies that were plotted as sigmoidal curves. Under the temporal differentiation 1–1.3 sec schedule, methamphetamine increased the frequency of short response durations at low doses, whereas high doses produced both long and short response durations, flattening the relative frequency distribution. Under the temporal differentiation 4–5.2 sec and 10–13 sec schedules, methamphetamine produced only short response durations, which shifted the relative frequency and cumulative frequency distribution of response durations leftward. Δ⁹-Tetrahydrocannabinol had little effect under the temporal differentiation 1–1.3 sec and 4–5.2 sec schedules, but it greatly increased the relative frequency of short response durations under the 10–13 sec schedule. Phencyclidine produced a similar effect under all temporal differentiation schedules, increasing the relative frequency of short response durations. Thus the effect of drugs on timing behavior under these temporal differentiation schedules not only depended on the drug, but also depended on the dose and the time parameters of the schedule. These data suggest that drugs produce multiple effects on timing behaviors that depend on complex interactions among several factors.