Upcoming food-pellet reinforcement alters rats' lever pressing for liquid sucrose delivered by a progressive-ratio schedule.

The present study investigated whether rats' responding for liquid-sucrose reinforcement delivered by a progressive-ratio (PR) schedule would be altered by the addition of food-pellet reinforcement available subsequent to the PR schedule. In Experiment 1, six rats lever pressed for 1% sucrose reinforcers delivered by a PR 3 schedule. In Experiment 2, six rats lever pressed for 5% sucrose delivered by a PR 5 schedule. In both experiments, baseline sessions consisted of 40min of exposure to the PR schedule. In the first treatment condition, a 25-min period of food-pellet reinforcement, delivered by a random-interval 60-s schedule, immediately followed the initial 40min. In the second treatment condition, the 25-min period of food-pellet reinforcement became available when 10min elapsed without the subject completing a ratio on the PR schedule. Results from both experiments showed that upcoming food-pellet reinforcement increased the number of ratios subjects completed on the PR schedule. Portions of the present results represent a partial replication of results reported by Baron and Derenne [J. Exp. Anal. Behav. 73 (2000) 291], who used a similar procedure. They also augment a growing body of research on positive induction.