Excitotoxic lesions of the gustatory thalamus eliminate consummatory but not instrumental successive negative contrast in rats

Lesions of the gustatory thalamus (GT) prevent the occurrence of between-session contrast effects (i.e., anticipatory negative contrast and consummatory successive negative contrast [cSNC]) involving liquid rewards. These deficits are attributed to a disruption of the reward comparison mechanism that computes the value of the current reward relative to the expected reward. Instrumental successive negative contrast (iSNC), which occurs following a surprising reduction in the magnitude of a solid food reward, is also keyed off the detection of a disparity between the value of the expected and actual rewards. The present study examined whether neurotoxic lesions of the GT prevent the occurrence of iSNC just as they abolish cSNC. In Experiment 1, both GT-lesioned and neurologically intact rats trained with 10 food pellets and shifted to one food pellet showed a significant iSNC effect. In Experiment 2, the same GT-lesioned rats failed to show cSNC when shifted from 1.0 M sucrose to 0.15% sodium saccharin. The obtained pattern of results suggests that separate reward comparison mechanisms with different neural substrates underlie expression of cSNC and iSNC.