Interevent anticipation of liquid and solid sucrose rewards

Abstract

Albino rats were trained in a straight runway under two different series of reward magnitude each day. In one three-run series (a trial), reward was given on the first and third runs (R-N-R), and in the other, reward was given only on the first run (R-N-N). The interval between trials was approximately 40 min, and the interval between runs within a trial was approximately 30 sec. For one group of rats, reward was 10 45-mg sucrose pellets. For the other group, reward was 1 ml of 30% liquid sucrose mixed by weight with tap water. With both reward types, running on the second run of a trial was faster if the third run was rewarded (R-N-R) than if it was not (R-N-N). This result suggests that the animals were in some way anticipating the reward amount for an upcoming run while running a current run. After 26 training days, the third run of the R-N-R trial was shifted to nonreward for 12 additional days. Times slowed on Runs 2 and 3 in the shifted trial, but there was no evidence of a successive negative contrast effect. The implications of the anticipation result for the interpretation of differences between sugar and ordinary food rewards in paradigms that suggest reward anticipation are discussed.