Quantitative representation of the upper surface of the human head.

A three dimensional digitizer was used to locate 62 positions on an electrode cap placed on the heads of nine subjects. Nonlinear regression methods were used to obtain the best fit sphere, ellipsoid, and perturbed ellipsoid. RMS errors varied from approximately 1 cm to 2 mm, depending on subject and surface chosen. The spherical coordinates of the standard 10/20 electrodes on best fit spheres were obtained. A method to estimate ellipsoid axes using only three standard EEG head dimensions with an ordinary tape measure is derived from the data. The results have at least two potential applications: the forward/inverse problem in EEG and MEG, and the estimation of surface Laplacians from EEG data.