the mastoid reference sites (TP9/10), however, it becomes obvious that recording sites along the reference-dependent isopotential line (e.g., T7/8, FT9/10, P9/10) showed little or no ERP activity. Thus, ERP activity at these sites is severely attenuated because of the arbitrary choice of the recording reference (Kayser & Tenke, 2006a, 2006b; Tenke & Kayser, 2005). In contrast, the reference-free CSD waveforms (black dashed lines) identified robust sink activity at these sites, which was not compromised by the choice of reference. Although the observed N1 sink and P2 source in the CSD waveforms directly corresponded to the N1 and P2 potentials in the OERP waveforms, marked topographic distinctions were evident, particularly with respect to the fronto-temporal N1 sink and lateral frontal sinks associated with the mid-parietal P2 source.4
