Effects of reward magnitude on rats' long-term spatial memory retention in the radial-arm maze.

Abstract

Our study examined whether reward magnitude affects long-term spatial memory retention in rats performing a radial-arm maze task. Previous studies, using a forced-choice learning phase followed by a free-choice test, have reported retention of up to 72 hours, but they typically used a large reward (10 pellets per arm; 45 mg each) rather than the standard reward (one pellet per arm), leaving open a reward-based explanation. In Experiment 1, we conducted a between-subjects comparison of two reward conditions: a standard-reward group (one pellet per arm) and a large-reward group (10 pellets per arm). The delay interval was progressively extended from 1 min to 1 hr, 24 hr, 48 hr, and 72 hr. Both groups performed significantly above chance at all delays, including 72 hr, and performance did not differ significantly between reward conditions. In Experiment 2, we tested whether relative changes in reward magnitude modulate retention. Within each reward group, reward magnitude was increased or decreased for only one arm during the learning phase, followed by a 24-hr retention test. Accuracy for the arm with the changed reward did not differ significantly from that of the unchanged arms. Together, these findings show that 72-hr retention can be obtained, even with the standard small reward (one pellet), and that radial-arm maze performance is robust to both absolute reward magnitude and relative changes in reward.