Lesions of mediodorsal thalamus and anterior thalamic nuclei produce dissociable effects on instrumental conditioning in rats

Two experiments examined the effects of bilateral excitotoxic lesions of either the mediodorsal (MD) or anterior (ANT) thalamic nuclei on instrumental acquisition and performance, sensitivity to changes in the value of the instrumental outcome, and sensitivity to changes in the instrumental contingency. Rats were food deprived and trained to press two levers, each earning a unique food outcome (pellets or sucrose). All rats acquired the instrumental response although ANT lesions appear slightly to increase and MD lesions slightly to suppress instrumental performance. After training, specific satiety-induced devaluation of one of the two instrumental outcomes produced a selective reduction in responding on the lever that in training had earned the now devalued outcome but only in the SHAM and ANT groups. In contrast, MD animals failed to show evidence of a selective devaluation effect when tested in extinction. Additionally, SHAM and ANT animals selectively decreased responding when one action-outcome contingency was degraded, whereas MD animals reduced responding nonselectively on the two levers. Subsequent tests established that an inability to discriminate between either the two actions or the two outcomes cannot account for the lack of selective responding observed in the MD animals. Together these data suggest that MD lesions produce a profound deficit in the ability of rats to utilize specific action-outcome associations and appear to render rats relatively insensitive to the causal consequences of their instrumental actions. In contrast, far from producing a deficit, ANT lesioned rats were as sensitive to the effects of these behavioural manipulations as the sham lesioned controls.