In their landmark textbook, Nunez and Srinivasan (2006) have stressed that compared to surface potentials, spatial band-pass filtering of distributed cortical sources reduces surface Laplacian estimates of large dipole layers in favor of smaller dipole layers. Forward simulations with a four-shell head model as a function of radial dipole layer size and different ratios of brain-to-skull conductivity revealed the maximum scalp potential for broad dipole layers (extending about 7–10 cm), whereas the surface Laplacian maximum was observed for small dipole layers (about 2.5 cm; cf. Figure 8–7 on p. 328 in Nunez and Srinivasan, 2006). However, these simulations were based on three-dimensional spline interpolations (Nunez and Srinivasan, 2006, Appendix J), which closely correspond to surface Laplacian estimates based on highly flexible spherical splines (m = 2–3; e.g., Perrin et al., 1989). Figure 15 shows these relations between cortical dipole layer size and EEG surface measures for two different brain-to-skull conductivity ratios. Scalp potentials were simulated for a fully-balanced, 81-channel 10–10 system EEG montage (Oostenveld and Praamstra, 2001) using a four-shell forward solution (Berg, 2006) with radial dipoles located 14 mm
