MEDIODORSAL THALAMIC LESIONS IMPAIR ACQUISITION OF AN EYEBLINK AVOIDANCE-RESPONSE IN RABBITS

Rabbits received ibotenic acid lesions of the mediodorsal nucleus of t he thalamus (MD) or sham lesions. These animals were compared on four sessions of instrumental avoidance conditioning, during which an eyebl ink (EB) response during the presentation of a tone-conditioned stimul us prevented the occurrence of a paraorbital electric shock unconditio ned stimulus. Lesions of MD retarded acquisition of the EB avoidance r esponse, but did not affect asymptotic performance. Concomitant heart rate (HR) changes were little affected by MD lesions, although there w as some evidence that such changes were slightly larger in MD-lesioned animals. These results suggest that MD participates in some general a spect of the learning process, perhaps by affecting other behavioral p rocesses such as 'attention' or 'arousal'.