Dissociating response selection and conflict in the medial frontal surface.

Response conflict and random response selection have both been associated with activations on the medial frontal surface. Random response selection was typically studied using a 'free selection' paradigm, in which subjects have to select between equally legitimate responses which are in competition with each other. Therefore, one interpretation is that the generation of spontaneous actions is merely a special case of the induction of response conflict. It has been reported that the generation of spontaneous actions is tightly associated with activity in the pre-supplementary motor area (pre-SMA). Most studies of response conflict, on the other hand, suggest that activations in a different and more ventral area, the anterior cingulate, reflect response conflict. Nonetheless, contradictory results have also been reported. To clarify this issue, in this experiment, we have put together a version of the Eriksen flanker task and a free selection task to allow for a direct comparison. We confirmed our hypothesis that free selection is associated with the pre-SMA, whereas response conflict is associated in the anterior cingulate. The two clusters of activation do not overlap, and the peaks of them were about 30 mm apart. We conclude that the activity in the pre-SMA is related to the endogenous generation of action rather than response conflict.