Within-session changes in responding during variable interval schedules.

Four rats and four pigeons responded for food delivered by variable interval schedules that provided programmed rates of reinforcement ranging from 15 to 480 reinforcers per hour. Rate of responding increased, decreased, or increased and then decreased within sessions. The within-session pattern of responding changed with changes in the programmed rate of reinforcement and with the species of subject. Finding within-session changes in responding during variable interval schedules extends the generality of these changes to another schedule. It implies that variable interval schedules should be used cautiously as baselines for assessing the effects of other variables, such as drugs. Finally, the results suggest that systematic pauses in responding during the session may contribute to the decreases in the average rates of responding with increases in the rates of reinforcement that are sometimes observed when subjects respond on variable interval schedules that deliver high rates of reinforcement.