Gustatory thalamus lesions eliminate successive negative contrast in rats: evidence against a memory deficit.

Successive negative contrast is the exaggerated reduction of licking that occurs when rats expecting a high-value reward are given a low-value reward. This effect is typically investigated with a 24-hr retention interval between access periods. The present experiment tested the hypothesis that the absence of successive negative contrast in rats with bilateral lesions of the gustatory thalamus (GT) is due to a memory deficit. The results argue against this hypothesis by showing that, irrespective of retention-interval duration (7.5 min, 15 min, 45 min, 180 min, or 24 hr), lesioned rats failed to show successive negative contrast. As such, the data are consistent with the alternative view that GT lesions specifically disrupt the reward comparison mechanism that underlies successive negative contrast.