Figure 2.

$R_j^{TEP}$ versus j for TEP electric fields. $R_j^{TEP}$ is the ratio of the mean-squared electric field over the scalp region to that over the brain region for an electric field comprised of a single vector spherical harmonic component indexed by the pair of integers j and m. The index j of the vector spherical representation of the electric field is reciprocally related to spatial scale in the field. Note that $R_j^{TEP}$ is independent of index m for any given value of j. The cost of greater electric field focality in the brain is greater mean-squared electric field in the scalp relative to the brain.