Reward Adaptation and the Mechanisms of Learning: Contrast Changes Reward Value in Rats and Drives Learning.

Formal theories of learning suggest that associations between events are determined by the internal representations of those events. Thus, learning should depend on perceived reward value-even when perceptions differ from objective values. We examined this prediction in flavor-preference learning in rats. In two experiments, simultaneous contrast either increased perceived reward value, which was paired with a distinctive flavor cue (the positive conditioned stimulus, CS+), or decreased the perceived value of the same reward, which was then paired with a second flavor (the negative conditioned stimulus, CS-). Even though the CS+ and CS- were paired with the same objective reward, there was a preference for the CS+ in subsequent tests. Moreover, the size of contrast-produced changes in reward value during training predicted the preference for the CS+ at test. This contrast-produced learning effect illustrates the mechanisms by which associations, which normally track veridical relationships between events in the world, are formed.