What can topology changes in the oddball N2 reveal about underlying processes?

A prominent theory of the N2 event-related potential component holds that the 'oddball' N2 is generated in the anterior cingulate cortex. However, observations of oddball N2s with posterior scalp distributions are inconsistent with this hypothesis. We suggest that variability in the topology of the oddball N2 is a key characteristic of the component that can inform theories of its neural basis. We propose that the oddball N2 reflects cortex-wide noradrenergic modulation of the ongoing cortical activity and thus should have a topology that varies systematically according to task specifics. Participants engaged in an oddball task with male and female faces tinted either yellow or blue, counting targets according to color or sex. Between blocks, targets were frequent or infrequent, counterbalanced across task (attend color, attend sex), and category (blue male, yellow male, blue female, yellow female). We created difference waves by subtracting frequent from infrequent category trials to isolate the oddball N2. When participants attended to color the oddball N2 was maximal over frontal-central areas and when they attended to sex it was maximal over lateral-occipital areas. Thus, the oddball N2 has a variable scalp distribution that depends on the relative engagement of cortical areas, consistent with noradrenergic modulation having the greatest impact in those areas mostly engaged by the task at hand.