Mediodorsal Thalamic Lesions Impair Trace Eyeblink Conditioning in the Rabbit

Abstract

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 primary thalamic nucleus providing projections to the prefrontal cortex, and, like the prefrontal cortex, has been implicated 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) response (Buchanan and Thompson 1990), but do not affect asymptotic performance. Similar small, although significant, MDN-lesion effects are seen on acquisition of an EB instrumental avoidance response (Buchanan 1994). Much more severe MDN-lesion effects are seen in a discrimination/reversal paradigm, particularly during the reversal phase. Thus, these lesions have little effect on the original acquisition of a discrimination between a reinforced conditioned stimulus (CS+) and a nonreinforced conditioned stimulus (CS−). However, if, after this discrimination has been acquired, the stimuli are reversed such that the previously reinforced CS+ is no longer reinforced and the previously nonreinforced CS− is now the CS+, MDN lesions dramatically impair acquisition of this reversal task (Buchanan 1991). Other investigators have reported similar findings (Gabriel et al. 1989). We have also shown that MDN damage significantly impairs EB conditioning when the interstimulus interval (ISI) is not optimal (i.e., a 2-sec ISI), or during partial reinforcement, when the schedule involves 25%, but not 100% or 50% reinforcement (Buchanan et al. 1997b). Further, multiple unit activity recorded from MDN during classical EB conditioning is correlated with acquisition of the conditioned response (CR) (Buchanan et al. 1997a), but such changes are relatively small and do not show a clear acquisition function.These findings suggest that MDN, or its inputs to PFC, may be important for acquisition of the classically conditioned EB response, although not essential. It appears that MDN function may be most critical under conditions of greater task complexity or difficulty, such as discrimination/ reversal or nonoptimal ISI or partial reinforcement conditions, but is less important for relatively simple tasks acquired under optimal conditions. Similar findings have been reported after damage to the hippocampus. Th...

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