The role of the nucleus accumbens in instrumental conditioning: Evidence of a functional dissociation between accumbens core and shell

In three experiments we examined the effect of bilateral excitotoxic lesion s of the nucleus accumbens core or shell subregions on instrumental perform ance, outcome devaluation, degradation of the instrumental contingency, Pav lovian conditioning, and Pavlovian-instrumental transfer. Rats were food de prived and trained to press two levers, one delivering food pellets and the other a sucrose solution. All animals acquired the lever-press response al though the rate of acquisition and overall response rates in core-lesioned animals were depressed relative to that in the shell- or sham-lesioned anim als. Furthermore, in shell- and sham-lesioned rats, post-training devaluati on of one of the two outcomes using a specific satiety procedure produced a selective reduction in performance on the lever that, in training, deliver ed the prefed outcome. In contrast, the core-lesioned rats failed to show a selective devaluation effect and reduced responding on both levers. Subseq uent tests revealed that these effects of core lesions were not caused by a n impairment in their ability to recall the devalued outcome, to discrimina te the two outcomes, or to encode the instrumental action-outcome contingen cies to which they were exposed. Additionally, the core lesions did not hav e any marked effect on Pavlovian conditioning or on Pavlovian-instrumental transfer. Importantly, although shell- lesioned rats showed no deficit in a ny test of instrumental conditioning or in Pavlovian conditioning, they fai led to show any positive transfer in the Pavlovian-instrumental transfer te st. This double dissociation suggests that nucleus accumbens core and shell differentially mediate the impact of instrumental and Pavlovian incentive processes, respectively, on instrumental performance.