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 below the outer scalp surface (85 mm head radius), expanding in ‘cap size’ from a focal dipole underlying Cz (0 cm) to a broad circle of 135° downward in steps of 3° (20 cm). Corresponding CSD estimates were calculated from these surface potentials using spherical spline interpolations with different flexibility constants (m = 2–7; λ = 10−5; Perrin et al., 1989). These simulations replicate those reported by Nunez and Srinivasan (2006) for both surface potentials (maxima at about 8–9 cm; Fig. 15A and B) and surface Laplacian estimates when using a high spline flexibility (i.e., m = 2 or m = 3; maxima at about 2.5–3 cm; red and green lines in Fig. 15C and D). They also confirm that the relative magnitudes of different spatial frequencies (i.e., the surface Laplacian transform acts as a spatial filter) are not affected by different brain-to-skull conductivity ratios. Most importantly, however, surface Laplacian estimates based on more rigid splines become gradually more sensitive
