Within-session rates of responding when reinforcer magnitude is changed within the session

In the present experiment, the authors investigated the idea that within-session changes in operant response rates occur because subjects sensitize and then habituate to the reinforcer. If that is true, then altering an aspect of the reinforcer within the session should alter the observed within-session responding. The authors tested that idea by having rats press a lever for 2 food-pellet reinforcers delivered by a variable-interval 120-s schedule during 60-min baseline sessions. In treatment conditions, the magnitude of the reinforcer was halved (1 pellet) or doubled (4 pellets) 10, 20, 30, 40, or 50 min into the session. That magnitude of reinforcement then remained in effect for the rest of the session. Altering reinforcer magnitude altered the rates of responding within the session in a fashion consistent with the habituation explanation, that is, response rates increased, relative to baseline, when the magnitude of reinforcement was increased. They decreased when the magnitude was decreased. Those results were seemingly inconsistent with the competing idea that within-session decreases in responding rates are produced by satiation.