Figure 6 compares N1 and P3 ERP topographies (using waveform peak amplitudes of visual word stimuli shown in Fig. 2A) with their surface Laplacian estimates stemming from this 3–5 nearest neighbors local Hjorth montage, as well as three additional local Hjorth differentiation grids using 8–9, 24–25 or 66 (i.e., all) nearest neighbors. Although overall amplitude and polarity of the ERP measures at each recording site differ substantially across different EEG references (Fig. 6A), their N1 and P3 topographies remain the same, revealing a negative maximum for N1 over left inferior-parietal sites and a positive maximum for P3 over mid-parietal sites.6 Consequently, these ERP topographies (NR, LM, AR, or any other reference scheme) will render identical local Hjorth estimates for any given differentiation grid (Fig. 6B). Depending on the number of nearest neighbors, the representation of surface potentials gradients will be more or less abrupt, with smoother transitions for increasing numbers of nearest neighbors. Whereas volume conduction results in smoother (or smeared) surface potential topographies, stretching any local minimum or maximum to neighboring scalp locations (cf. isopotential lines in Fig. 6A),
