Likewise, P3a amplitudes and latencies at Cz for novels also differed between EEG references, peaking somewhat earlier for ERPs referenced to nose or linked mastoids (320 ms and 325 ms) compared to P3b, but considerably earlier for the average reference (260 ms; Fig. 11B). Still, a notable deflection can also be seen at this latency in the nose- and linked-mastoids ERPs. Again, all P3a ERP topographies show an identical broad scalp distribution with a vertex maximum (Fig. 11E, left columns). In sharp contrast, the corresponding CSD waveforms had a distinct positive maximum at Cz (255 ms), with sources extending laterally and posteriorly along mid-parietal sites (Fig. 11E, right column). Considering the time course (i.e., early and late P3 intervals) of both conditions (target, novel) together, the surface Laplacian estimates reveal two distinct positive components having separate scalp distributions (Fig. 11C–F, right column). The CSD equivalent of the P3a, termed novelty novel vertex source (NVS) on the basis of its functional and topographic properties, is a reliable phenomenon across different healthy and psychiatric populations (Kayser et al., 2014; Tenke et al., 2010). Obviously, no such clear distinction between P3a and P3b can be made on the basis of surface potentials.
