Parasagittal thalamic knife cuts retard Pavlovian eyeblink conditioning and abolish the tachycardiac component of the heart rate conditioned response.

Rabbits received parasagittal knife cuts lateral to the mediodorsal nucleus of the thalamus (MD), severing afferents and efferents to and from the prefrontal cortex. These animals were compared to sham animals in a Pavlovian eyeblink and heart rate conditioning experiment in which a tone was the conditioned stimulus (CS) and paraorbital electrical shock was the unconditioned stimulus (US). Knife cuts retarded acquisition of the eyeblink conditioned response (CR), and abolished the late-occurring tachycardiac component of the heart rate CR. These data are compatible with previous experiments which suggest that MD participates in the sympathetic control associated with somatomotor learning.