When 'go' and 'nogo' are equally frequent: ERP components and cortical tomography.

In human electrophysiology, a considerable corpus of studies using event-related potentials have investigated inhibitory processes by employing the 'go-nogo' paradigm, which requires responding to one type of event while withholding the response to another type of event. Two event-related potential waveform features (N2 and P3) have been associated with larger amplitude in nogo trials than in go trials. Traditionally, these differences were thought to reflect response inhibition. Recently, the source localization of N2 to the anterior cingulate cortex, as well as the colocalization of N2 with error-related negativity, has been interpreted in terms of conflict monitoring. In order to isolate the contribution of inhibitory processes, we matched the frequency of the go and nogo events, thus minimizing differences in response conflict between event types. A data-driven analytical procedure contrasted go with nogo events across the entire event-related potential segment and found that N2 reliably differentiated between the two conditions while P3 did not. Tomographical analyses of the N2 difference observed in conditions of equal go and nogo trial frequency localized N2 to the right ventral and dorsolateral prefrontal cortex. Because a growing body of evidence implicates these brain regions in inhibitory processes, we conclude that N2 does, at least in part, reflect inhibition.