The scalp-based surface Laplacian is distinguished from a three-dimensional CSD, being limited to the external surface that encompasses the complete volume-conduction model. In fact, Nunez and Srinivasan (2006a) have urged against unifying implementations of an intracranial CSD and a surface Laplacian, arguing that a “different principle” is involved, because the accuracy and utility of scalp-based surface Laplacian depends on the “focusing of cortical currents by the high-resistivity of the skull.” Despite these distinctions, Eq. 4 still reflects Poisson’s source equation for scalp data. The surface Laplacian therefore identifies the locations at which current is injected into the measurement space (i.e., the scalp recording montage) from the underlying multi-shelled conductive media.
