To investigate the degree to which the ACC is involved in voluntary response selection, in performance monitoring, or in assessing the outcome of people's own decisions, Walton and colleagues carried out an fMRI study which parametrically manipulated the necessity to choose a response and to monitor its consequences (Walton, Devlin and Rushworth, 2004). In this task, people were taught 3 sets of conditional response rules which linked 3 stimuli with 3 possible finger movements (Figure 2a). On each trial, one of the stimuli would be presented and the participants' task would be to make an appropriate response based on which response rule was currently in place. Feedback was provided after each response as to whether or not the correct action had been selected. After a number of trials on which participants responded according to one particular response rule, a cue appeared either instructing them to stay using the same rule or to switch to one of the other two sets of response rules. However, unlike in most previous comparable experiments, the switch cue only informed subjects that a change had occurred but did not instruct which set of response rules was now in place.
