Such a finding concurs with recent studies showing that neurons in the ACCs appear to encode the average probabilistic value of available rewards (Amiez, Joseph and Procyk, 2006). Interestingly, in this study by Amiez and colleagues, where monkeys had to learn which of two stimuli with probabilistic associations with reward was optimal to choose, inactivation of part of ACCs again caused impairments in using reward information to guide behaviour. However, in contrast with the lack of persistence observed in the studies by Kennerley and colleagues (2006) and by Shima and Tanji (1998), here the monkeys seemed liable to persist with the option selected on the first trial, regardless of whether it gave a large or small reward. One possible reason for this discrepancy is that while the outcome on each trial was uncertain, the long-term values of the stimuli in the study by Amiez and colleagues remained stable. By contrast, both other studies involved long-periods of training on a response switching task. While the ACCs may be vital for learning about the overall value of actions, other regions such as
