WITHIN-SESSION CHANGES IN RESPONDING DURING CONCURRENT VARIABLE-INTERVAL SCHEDULES

Five rats and 4 pigeons responded for food delivered by several concur rent variable-interval schedules. The sum of the rates of reinforcemen t programmed for the two components varied from lj to 480 reinforcers per hour in different conditions. Rates of responding usually changed within the experimental session in a similar manner for the two compon ents of each concurrent schedule. The within-session changes were simi lar to previously reported changes during simple schedules that provid ed rates of reinforcement equal to the sum of all reinforcers obtained from the concurrent schedules. The number of changeovers also changed within sessions in a manner similar to the changes in instrumental re sponding. These results suggest that changeovers are governed by the s ame variables that govern instrumental responding. They also suggest t hat the within-session change in responding during each component of a concurrent schedule is determined by approximately the sum of the rei nforcers obtained from both components when both components provide th e same type of reinforcer.