EFFECTS OF CHLORPROMAZINE ON FIXED‐RATIO RESPONDING: MODIFICATION BY FIXED‐INTERVAL DISCRIMINATIVE STIMULI

Effects of chlorpromazine (1 to 100 mg/kg) were assessed on two pigeons' responding under various modifications of a multiple schedule of food delivery. During a fixed‐interval component, the first response after 5 min produced food; during the subsequent, fixed‐ratio component, the 30th response produced food. Modifications of the schedule entailed changes in stimulus conditions imposed during the fixed‐ratio component that did not systematically alter characteristics of performance under non‐drug conditions. In the first phase of the experiment, distinctive visual stimuli were correlated with each schedule component (conventional multiple schedule); chlorpromazine produced small decreases in fixed‐ratio responding (20% at 30 mg/kg). When each response during the fixed‐ratio component produced the stimulus correlated with the fixed‐interval schedule (fixed‐interval discriminative stimulus) for 1.2 s, effects of chlorpromazine were not different from those under the conventional multiple schedule. Chlorpromazine produced greater decreases in fixed‐ratio responding (55% at 30 mg/kg) when either the first response of each fixed ratio changed the stimulus correlated with the fixed‐ratio schedule to the fixed‐interval discriminative stimulus for the remainder of the fixed‐ratio component, or when the fixed‐interval discriminative stimulus was presented independently of responding according to a matched temporal sequence. When the fixed‐interval discriminative stimulus was present continuously during the fixed‐ratio component (mixed schedule), chlorpromazine produced even more substantial decreases in fixed‐ratio responding (greater than 80% at 30 mg/kg). Effects of chlorpromazine on fixed‐interval responding were also modified by the schedules of fixed‐interval discriminative stimulus presentation. The effects of chlorpromazine were a joint function of the stimuli prevailing during the multiple schedule and the degree to which responding influenced these stimuli.