As with the local Hjorth algorithm, the surface Laplacian estimate at any recording site (or intermediate surface location) depends only on the data measured at that site in relation to the potentials measured all other sites (i.e., the ‘neighbors’), with their impact weighted by their (spherical) distance to the site (surface location) under consideration. Thus, while all recording sites affect the surface Laplacian estimate at any surface location, their influence will depend on the specific surface location. In fact, for a given spline flexibility (and other fixed computation parameters discussed below), the (spherical) distances between the to-be-estimated CSD location and all EEG recording sites are the sole determinants for computing the CSD estimate from the surface potentials. Because all electrode sites are assumed to be on the scalp surface and the scalp is modeled as a sphere, the distance (in mm) between electrode locations is proportional to their angular distance, which is readily expressed by the cosine of the angle between any two surface points (i.e., radians). For example, for a unit sphere (radius r = 1), the spherical distance
