Induction when rats respond for liquid-sucrose reinforcement as a function of amount of upcoming "work"

Previous research has shown that rats increase their rate of responding for 1% sucrose reinforcement in the first half of the session if food-pellet, rather than 1% sucrose, reinforcement will be available in the second half (i.e., positive induction). Four experiments investigated whether this induction effect would be changed by altering the amount of "work" required to earn reinforcement in the second half of the session. In Experiments 1, 2, and 3, rats pressed a lever for 1% sucrose delivered by a random-interval 60-s schedule during the first 25 min of the session. In different conditions, the reward in the second half was either 1% sucrose or a food pellet delivered on a fixed-ratio (FIR) 10, 20, or 40 schedule. The FR schedule was in effect throughout the second half of the session (Experiment 1), until 25 reinforcers had been collected and then extinction was in effect (Experiment 2), or until 25 reinforcers had been collected and then the session ended (Experiment 3). Positive induction occurred in each experiment, but did not vary as a function of upcoming FR requirement. Experiment 4 yoked the rate of reinforcement on a random-interval schedule in the second half of the session to that obtained when subjects responded on a random-ratio schedule. Similar induction effects were observed regardless of which schedule was in effect in the second half of the session. These results suggest induction is primarily controlled by the availability of upcoming differential reinforcement, not by upcoming "work."