Figure 1.

The three-shell spherical head model in which the regions from outermost to innermost are the scalp, skull and brain respectively. Typical estimates for three shell model radii are r₀ = 80 mm, r₁ = 86 mm and r₂ = 92 mm whereas typical estimates σ₀ = σ₂ and ϵ = σ₁/σ₀ = 1/80. Note that only the ratio ε, rather than the specific values of the conductivities, is of importance in this work since the focus is on the calculation of mean-squared electric field ratios between the brain and scalp regions. The mean-squared electric field is a single number proxy for the energy dissipation in a region whereas it is square root is a single number proxy for the electric field amplitude in a region.