Ratio of scalp measure to cortical potential as a function of dipole
layer size. A, B: Vertex scalp potentials for a 81-channel 10–10 system
EEG montage (average reference) due to simulated dipole layers of varying
angular extent, forming superficial spherical caps in a four-shell forward
solution (Berg, 2006) for two different
ratios of brain-to-skull conductivity with the following conductivities [S/m]:
brain = 0.33, skull = 0.0042 (A: ratio = 78.6) or 0.0084 (B: ratio = 39.3), CSF
= 1.0, scalp = 0.33. C, D: Corresponding surface Laplacian estimates (CSD:
current source density) using spherical splines of different flexibility
(m = 2–7; λ = 10−5;
Perrin et al., 1989). Note that
surface potentials are primarily sensitive to broad dipole layers, whereas the
sensitivity of spherical spline surface Laplacian estimates varies with spline
flexibility.