Role of the nucleus accumbens core and shell in Pavlovian–instrumental transfer

When an initially neutral stimulus has been paired in Pavlovian fashion with an appetitive outcome such as food, noncontingent presentation of this conditioned stimulus (CS) can enhance ongoing instrumental responding, a phenomenon termed Pavlovian–instrumental transfer (PIT). In its simplest form, PIT is assessed by presenting a CS for food while the subject is responding in extinction on a single lever for the same food. It has previously been shown that the nucleus accumbens, and particularly the core subdivision, is critical for this form of PIT (Hall et al., 1999). However, behavioural studies have shown that PIT can be subdivided into a general, motivating effect of the Pavlovian CS, and a response-specific PIT effect, seen as a further enhancement when the Pavlovian and instrumental outcomes are the same (see Dickinson & Balleine, 1994). In the present study, rats received lesions to the core or shell of the nucleus accumbens before being tested on a response-specific PIT task. In the Pavlovian phase, one stimulus, CS(pel), was paired with pellet delivery, while a second stimulus, CS(suc), was paired with sucrose solution. The subjects were then trained to respond on two levers, with one lever producing pellets and the other producing sucrose. On test, lever-pressing was recorded in extinction while the stimuli were presented noncontingently. Control subjects (n = 6) showed a selective enhancement of lever-pressing on the lever paired with the same outcome as the Pavlovian CS; this is the response-specific PIT effect. Core-lesioned subjects (n = 4) showed a general enhancement of responding during the CS, but this was not specific to one response. Shell-lesioned animals (n = 4) showed no PIT. It is suggested that in this task, the shell is required for the ‘vigour’ and the core for the ‘direction’ of the potentiation of responding by a noncontingent, appetitive stimulus. This pattern closely resembles that previously observed for the potentiation of responding for conditioned reinforcement by psychostimulant drugs injected into the nucleus accumbens (Parkinson et al., 1999b).