Memory for magnitude of reinforcement: dissociation between the amygdala and hippocampus

Rats were trained on a successive delayed conditional discrimination task measuring memory for magnitude of reinforcement. In the study phase of the task, the rats were given one of two cereals. One cereal contained 25% sugar; the other 50% sugar. One of the two cereals was always designated the positive stimulus and the other the negative stimulus. This study phase was followed by the test phase in which the rat was shown an object which covered a food well. If the rat was given the negative stimulus in the study phase of the trial, no food reward was placed beneath the object. Whenever the positive stimulus was presented a food reward was available beneath the object. Performance was measured as the latency to uncover the food well. After reaching criterion level, the rats were given amygdala, hippocampal, or control lesions. Amygdala-lesioned rats showed significant deficits in performance, whereas no long-term deficits were observed for the hippocampal-lesioned groups even at longer retention delays. In additional experiments, it was shown that amygdala-lesioned, like normal, rats had similar taste preferences. Finally, normal and hippocampal-, but not amygdala-, lesioned rats transferred readily to different cereals containing 25% or 50% sugar. Thus, it appears that the amygdala, but not the hippocampus, plays a significant role in explicit data-based or working memory for affect information based on magnitude of reinforcement.