site Cz is estimated from the surface potentials measured at Cz and its four nearest neighbors (C1, C2, FCz, CPz). Given that these neighbors have all the same distance d to the Cz, owing to the fact that these are standard 10–10 system locations (e.g., Jurcak et al., 2007), the surface Laplacian (local Hjorth H) at site Cz is computed from the observed potentials P as HCz = PCz - (PC1/d + PC2/d + PFCz/d + PCPz/d) / 4d = PCz - (PC1 + PC2 + PFCz + PCPz) / 4. Accordingly, the local Hjorth derivation for the entire 67-channel EEG montage can be conveniently defined via a channel-by-channel transformation matrix, with the diagonal consisting of ones (+1.0) and the other columns in each row representing the respective (negative) weights for each neighboring site, or zeros if not included in the set of nearest neighbors for a given site; the local Hjorth estimates simply corresponds to the rowwise sums of the potentials measured at each site multiplied by the column weights (an analogous formalization can be easily defined in the one-dimensional case via a transformation vector). Because this transformation matrix is independent of the actual EEG signal (i.e., the values
