left hand responses, and to the left hemisphere (cf. location 13, approximately FC1) for right hand responses (Fig. 13A, column 2 and 3; Fig. 13B, column 4). These unilateral midfrontal sinks are accompanied by bilateral centroparietal sources, which are also more distinct over the hemisphere contralateral to the response hand (cf. location 94, approximate CCP4, for left press; location 43, approximate CCP3, for right press). In contrast, the response-locked ERPs reveal almost no midfrontal negativity, although minor deflections are detectable for linked-mastoids and average references at locations 107 and 13. Much of the response-locked ERP scalp topography is characterized by a broad positivity resembling a parietal P3b, particularly for the nose reference because the most negative potential is at the nose. Still, asymmetric shifts can nevertheless be deduced in this topography from the isopotential lines, which are consistent with a frontal negativity contralateral to the response (again, the topographies for each response hand do not differ between references).
