Olfactory learning in rats with lesions of the thalamic mediodorsal nucleus

The olfactory function of the mediodorsal thalamic nucleus (MD) in rats was investigated. After initial training on a simple odor detection task, experimental rats received lesions which destroyed the medial (olfactory) half MD while controls received cortical lesions or no lesions. Rats were tested postoperatively on detection, a series of 13 2-odor discriminations, and a reversal of the 13th discrimination, in which the previous S+ (positive stimulus) became S$-$ (negative stimulus) and the previous S$-$ became S+. All rats had a good retention of the detection. Controls, but not rats with MD lesions, rapidly acquired an olfactory learning set. The performance of experimental rats was quite variable and most failed to learn one or more postoperative tasks. Nevertheless, several rats with complete destruction of the medial half MD did improve over the series of odor discriminations and, hence, the lesion does not completely preclude acquisition of an olfactory learning set.