Isolation of saccade inhibition processes: rapid event-related fMRI of saccades and nogo trials.

Previous functional magnetic resonance imaging (fMRI) studies have compared saccade trials and nogo trials, which required subjects to look at peripheral visual stimuli and to inhibit automatic saccades evoked by peripheral stimuli, respectively. These studies surprisingly reported no activation differences in cortical saccade regions between the two tasks, despite their opposite response requirements. Here, we re-examined this issue by comparing saccades and nogo trials using a rapid event-related fMRI design in which saccade trials were presented twice as frequently as nogo trials to make the saccade response more prepotent. We hypothesized that this should increase recruitment of response inhibition processes in the nogo task, thereby increasing fMRI activation on nogo trials. Saccade and nogo trials were presented in whole and half trial versions. Whereas whole trials included a trial type instruction followed by peripheral stimulus presentation and subject response, half trials included only the instruction component, allowing us to measure instruction-related activation separately from response-evoked signals for both saccades and nogo trials. Instruction-related activation was greater for nogo versus saccade trials in right frontal eye field, middle frontal gyrus, intraparietal sulcus, and precuneus, which we attribute to a mixture of preparatory and task switching processes. Response-related activation was greater for nogo trials in supplementary eye field, anterior cingulate cortex, inferior frontal gyrus, and right supramarginal gyrus, and we attribute these results to saccade inhibition and other processes associated with an increased requirement for inhibition of the automatic saccade in nogo trials.