Positive contrast obtained in rats following a shift in schedule, delay, and magnitude of reward

Abstract

Sixty-four rats were used in a 2 by 2 by 2 factorial design (Phase 1) which combined two levels of delay schedule (partial vs. continuous), two levels of magnitude of reward (1 vs. 22 pellets), and two levels of delay of reward (10 vs. 30 sec). On each trial in Phase 2, each subject received 22 pellets after a 10-sec delay interval. A significant positive contrast effect was obtained when all three variables of magnitude, delay, and schedule were upshifted. Similarly, positive contrast was obtained based on a shift in delay and schedule, magnitude and schedule, and magnitude and delay. Moreover, positive contrast was obtained based on either an upward shift in schedule or magnitude. A simple decrease in delay of reward did not produce positive contrast. The results were viewed as favoring relative rather than absolute theories of reinforcement. However, a modified sequential (capaldian) interpretation of positive contrast was also offered.