Search

In three experiments we examined the effect of bilateral excitotoxic lesions of the nucleus accumbens core or shell subregions on instrumental performance, outcome devaluation, degradation of the instrumental contingency, Pavlovian conditioning, and Pavlovian-instrumental transfer. Rats were food deprived and trained to press two levers, one delivering food pellets and the other a sucrose solution. All animals acquired the lever-press response although the rate of acquisition and overall response rates in core-lesioned animals were depressed relative to that in the shell- or sham-lesioned animals. Furthermore, in shell- and sham-lesioned rats, post-training devaluation of one of the two outcomes using a specific satiety procedure produced a selective reduction in performance on the lever that, in training, delivered the prefed outcome. In contrast, the core-lesioned rats failed to show a selective devaluation effect and reduced responding on both levers. Subsequent tests revealed that these effects of core lesions were not caused by an impairment in their ability to recall the devalued outcome, to discriminate the two outcomes, or to encode the instrumental action-outcome contingencies to which they were exposed. Additionally, the core lesions did not have any marked effect on Pavlovian conditioning or on Pavlovian-instrumental transfer. Importantly, although shell-lesioned rats showed no deficit in any test of instrumental conditioning or in Pavlovian conditioning, they failed to show any positive transfer in the Pavlovian-instrumental transfer test. This double dissociation suggests that nucleus accumbens core and shell differentially mediate the impact of instrumental and Pavlovian incentive processes, respectively, on instrumental performance.

Considerable evidence suggests that, in instrumental conditioning, rats can encode both the specific action-outcome associations to which they are exposed and the degree to which an action is causal in producing its associated outcome. Three experiments assessed the involvement of the hippocampus in encoding these aspects of instrumental learning. In each study, rats with electrolytic lesions of the dorsal hippocampus and sham-lesioned controls were trained while hungry to press two levers, each of which delivered a unique food outcome. Experiments 1A and 1B used an outcome devaluation procedure to assess the effects of the lesion on encoding the action-outcome relationship. After training, one of the two outcomes was devalued using a specific satiety procedure, after which performance on the two levers was assessed in a choice extinction test. The lesion had no detectable effect on either the acquisition of instrumental performance or on the rats' sensitivity to outcome devaluation; lesion and sham groups both reduced responding on the lever associated with the devalued outcome compared with the other lever. In experiment 2, the sensitivity of hippocampal rats to the causal efficacy of their actions was assessed by selectively degrading the contingency between one of the actions and its associated outcome. Whereas sham rats selectively reduced performance on the lever for which the action-outcome contingency had been degraded, hippocampal rats did not. These results suggest that, in instrumental conditioning, lesions of the dorsal hippocampus selectively impair the ability of rats to represent the causal relationship between an action and its consequences.