Amygdaloid complex lesions differentially affect retention of tasks using appetitive and aversive reinforcement

The hypothesis that the amygdaloid complex (AC) is involved in the formation of stimulus-reward associations was examined. A series of experiments (1A-1C) directly compared the effects of lesions (produced by injection of the excitotoxin N-methyl-D-aspartic acid) on 1-trial appetitive and 1-trial aversive learning in rats. Experiments 1A and 1B, which used different degrees of reinforcement, revealed no effect of lesions on the appetitive task, whereas acquisition of the aversive task was significantly impaired. This impairment depended on the nature of the aversive reinforcement used: Impairment was seen when a highly aversive stimulus (footshock) was used but not when a less aversive stimulus (0.2% quinine solution) was used. Control experiments indicated that the effect of lesions was not due to reduced sensitivity to the foot-shock. In Experiment 2, a novel odor conditioning task examined further the effect of AC lesions on the acquisition of appetitive and aversive stimulus-reinforcement associations. As in Experiment 1, the AC lesions impaired learning of the aversive association but did not significantly influence the appetitive association. It is argued that, although the AC may be involved in some types of appetitively rewarded learning, the findings of a differential effect of AC lesions on aversively rewarded learning suggest a role in learning beyond the formation of stimulus-reinforcement associations.