Among the strongest arguments against the (exclusive) use of the surface Laplacian is the notion that EEG signals of low spatial frequency (i.e., originating from deep and/or distributed generator sources) are suppressed by a spatial high-pass filter (e.g., Nunez et al., 1997, 1999). For simulated radial and tangential dipoles, Perrin et al. (1987) plotted the different attenuation (fall-off) of CSDs and field potentials as a function of generator distance d (eccentricity) from the scalp surface. This attenuation is sharper for CSDs (1/d3 versus 1/d), indicating that the surface Laplacian emphasizes shallow, cortical generators. However, in this recently reproduced plot (Giard et al., 2014), both attenuation curves were adjusted by their respective peak maxima, ergo allowing no conclusions about how well (or reliable) each measure represents or “sees” brain activity of any given deep or shallow generator (i.e., without a direct, cross-measure comparison). Stated differently, this comparison between SP and SL measures implicitly assumes that 1) both measures are equally good in reflecting the signal maximum, and 2) larger values (or values per se) reflect a signal.
