Abstract

The effects of d-amphetamine, pentobarbital and chloropromazine were determined on the rate of conditioned key pecking of pigeons under multiple and mixed fixed-ratio 30 fixed-interval 10-minute schedules of food presentation. Under the multiple schedule, blue and red key lights were illuminated during the ratio and interval components, respectively, while a white key light was illuminated during both components of the mixed schedule. Pentobarbital produced comparable decreases in fixed-ratio response rates under the mixed and multiple schedules. Responding under the fixed-interval component of the multiple schedule was suppressed at lower doses than responding under the mixed fixed-interval component. d-Amphetamine decreased mixed fixed-ratio response rates more than multiple fixed-ratio response rates. Multiple and mixed fixed-interval response rates were increased by low doses of d-amphetamine but decreased by higher doses. At the highest dose, the responding in both components under the mixed schedule was more suppressed than the responding under the multiple schedule. Chlorpromazine markedly decreased responding under mixed fixed-ratio components, while only slightly decreasing responding under multiple fixed-ratio components. Responding in fixed-interval components was equally suppressed by chlorpromazine under both multiple and mixed schedules. All three drugs exhibited rate-dependent effects within the fixed-interval components, increasing the low rates of responding early in the fixed interval and decreasing the higher rates of responding in the terminal portions of the fixed interval, under both the multiple and mixed schedules. The control rate below which low rates were increased in the fixed interval decreased as a function of dose for all three drugs, but differed across drugs.