The EEG and MEG, Using a Model of Eccentric Spheres to Describe the Head

Two spherical models describing the head are used to compute EEG´s and MEG´s for current dipolar sources located in the visual cortex area. In one of the models, the spheres are concentric, in the other eccentric, fitting the compartment boundaries of the head best in the source area. In forward computations, a strong dependency on the volume conductor model is found for the EEG´s, but not for the MEG´s. The dependency of the EEG due to realistic values of the eccentricities of the spheres is in accordance with experiments described in the literature. All spheres are obtained by means of a least squares fitting algorithm in which the points on the surface boundaries, used to fit the sphere, may move within an interval determined by the measuring inaccuracy. This strategy used in the fitting algorithm is shown to give considerably better results than the one normally used.