Research Mediodorsal Thalamic Lesions Impair Trace Eyeblink Conditioning in the Rabbit

Rabbits received lesions of the mediodorsal nucleus of the thalamus (MDN) or sham lesions and were subjected to classical eyeblink (EB) and heart rate (HR) conditioning. All animals received trace conditioning, with a.5-sec tone conditioned stimulus, a.5-sec trace period, and a 50-msec periorbital shock unconditioned stimulus. Animals with MDN lesions acquired the EB conditioned response (CR) more slowly than sham-lesioned animals. However, previous studies have shown that MDN damage does not affect delay conditioning using either.5-sec or 1-sec interstimulus intervals. The lesions had no significant effect on the HR CR. These results suggest that information processed by MDN and relayed to the prefrontal cortex is required for somatomotor response selection under nonoptimal learning conditions. The mediodorsal nucleus of the thalamus (MDN) is a pri-mary thalamic nucleus providing projections to the prefron-tal cortex, and, like the prefrontal cortex, has been impli-cated in various aspects of learning and memory. We have shown previously that lesions of MDN slightly retard the acquisition of a classically conditioned eyeblink (EB) re-sponse (Buchanan and Thompson 1990), but do not affec