Fig. 1.

Simulations of sensitivity of potentials and surface Laplacian to dipole sources. Simulations were performed using a “4-sphere” model of the head. The spherical surfaces are (1) brain, (2) cerebrospinal fluid (CSF), (3) skull, and (4) scalp. Potentials and Laplacians are calculated on the outer (scalp) surface. Our standard model parameters are the radii of the spheres (r₁ = 8.0 cm, r₂ = 8.1 cm, r₃ = 8.6 cm, r₄ = 9.2 cm) and the conductivity ratios between the spherical shell regions (σ₁₂ = 0.2, σ₁₃ = 40, σ₁₄ = 1). The only parameter varied in these simulations is the brain-to-skull conductivity ratio (σ₁₃) from 20 to 160. (a) Dependence of potential on source depth for radial and tangential dipoles. Depth is calculated from the top of the brain sphere (r₁ = 8.0). (b) Dependence of surface Laplacian on source depth. (c) Dependence of outer surface (scalp) potential on the spatial extent of dipole layers composed of superficial radial dipole sources at a fixed radial location (r = 7.8). (d) Dependence of the Laplacian on the spatial extent of dipole layers composed of superficial radial dipole sources at a fixed radial location (r = 7.8).