Prominent stimulus-locked visual CSD components (Figure 3) included inferior lateral-parietal P1 sources (approximate peak latency 80 ms at P8 in controls) and N1 sinks (145 ms at P7), followed by an occipital P2 source (210 ms at Oz), a central N2 sink (270 ms at Cz), and mid-parietal P3 sources (500 ms at Pz). In contrast to nose-referenced ERPs, which showed a visual N2 maximum over the left inferior-parietal region, the reference-free CSDs revealed a vertex maximum for a visual N2 sink, indicating that the true origin of the underlying N2 generator is substantially masked by volume-conduction when using a nose reference. Like its ERP counterpart, however, the vertex N2 sink was severely diminished in patients compared with controls, and earlier N1 sink activity was also reduced in patients over left occipital-parietal sites (at O1, P7). Furthermore, an extended late source at Cz in controls that followed the N2 sink was almost absent in patients. In contrast, P3 source amplitude was comparable in controls and patients at site P3 where it was greatest.
