Mediodorsal thalamic lesions and Pavlovian conditioning of heart rate and eyeblink responses in the rabbit.

Rabbits received ibotenic acid lesions of the mediodorsal nucleus of the thalamus (MD) or sham lesions. These animals were compared on 4 sessions of Pavlovian eyeblink and heart rate conditioning, in which a tone was the conditioned stimulus and a paraorbital electrical shock was the unconditioned stimulus. Lesions of MD retarded acquisition of the eyeblink conditioned response and abolished the late-occurring tachycardiac component of the heart rate conditioned response. The data are compatible with previous experiments suggesting that MD participates in the sympathetic control associated with somatomotor learning.