Based on the topographic and temporal distinction between the novelty P3 and target P3b in the same task, spatial PCA methods have confirmed the characteristic midline frontocentral topography of novelty P3 (Spencer et al., 2001), as does cortical image transformation (deblurring; He, Lian, Spencer, Dien, & Donchin 2001) and the related, but simpler, surface Laplacian (Friedman et al., 2001). When probed using intracranial electrodes (oddball task; nose reference), P3a typically appears as part of the triphasic component sequence, N2a/P3a/SW (Halgren et al., 1995). In the vicinity of the posterior cingulate gyrus, the potential gradients are steep, suggestive of proximity to a generator. However, the same component sequence is seen within the frontal lobe as well (Baudena, Halgren, Heit, & Clarke, 1995), but with no significant differences between distracters and targets, and is characterized by steep intracranial field potential gradients near the inferior frontal sulcus. Findings more consistent with the scalp topography of the novelty P3 are the large amplitudes (> 100 μV) and steep gradients in orbitofrontal regions, with polarity inversions at the most medial sites. In a summary of
