A Method for Locating Scalp Electrodes in Spherical Coordinates

Volume conduction models of the human head have often been utilized in relating scalp potential measurements to the locus of the underlying neural sources. However, the determination of the source loci depends on the accurate location of electrodes in a geometry suited to the form of the modeling equations. This paper describes a method for fabricating a Plexiglas helmet that reproducibly locates a large number of electrodes. The electrodes are laid out according to a "latitude and longitude" spherical angular system which is defined by the geometry of the individual subject\´s head. This coordinate system is appropriate for localization techniques which model the head as a spherically symmetric volume conductor. Helmets for eight subjects were measured to show that the deviations from sphericity over the back of the head are small, on the order of ±3 percent. The merits of this approach are discussed in comparison to an ellipsoidal model suggested by Schneider (1972).