Action selection: A race model for selected and non-selected actions distinguishes the contribution of premotor and prefrontal areas

Race models have been used to explain perceptual, motor and oculomotor decisions. Here we developed a race model to explain how human subjects select actions when there are no overt rewards and no external cues to specify which action to make. Critically, we were able to estimate the cumulative activity of neuronal decision-units for selected and non-selected actions. We used functional magnetic resonance imaging (fMRI) to test for regional brain activity that correlated with the predictions of this race model. Activity in the pre-SMA, cingulate motor and premotor areas correlated with prospective selection between responses according to the race model. Activity in the lateral prefrontal cortex did not correlate with the race model, even though this area was active during action selection. This activity related to the degree to which individuals switched between alternative actions. Crucially, a follow-up experiment showed that it was not present on the first trial. Taken together, these results suggest that the lateral prefrontal cortex is not the source for the generation of action. It is more likely that it is involved in switching to alternatives or monitoring previous actions. Thus, our experiment shows the power of the race model in distinguishing the contribution of different areas in the selection of action.