Altering Reinforcer Variety or Intensity Changes the Within-Session Decrease in Responding

Operant responding often changes systematically within experimental sessions. McSweeney, Hinson, and Cannon (1996) argued that sensitization and habituation produce within-session changes in responding. The present study tested two predictions of the sensitization–habituation explanation. In two experiments, rats pressed a lever for reinforcers delivered by a multiple variable interval 15-s variable interval 15-s schedule. In Experiment 1, the variety of reinforcers delivered during the session was manipulated by varying the percentage of programmed reinforcers replaced with qualitatively different reinforcers from 0 to 75%, in five different conditions. In Experiment 2, the intensity of the reinforcer was manipulated by varying the concentration of sucrose in the sucrose and water solution used as the reinforcer from 0 to 30%, in five different conditions. Increasing the variety or the intensity of the reinforcers slowed the within-session decrease in responding. The results are consistent with the predictions of a sensitization–habituation explanation of within-session changes in responding.