The capacity of a surface Laplacian to separate individual components based on differences in topography and time course is exemplarily shown for an auditory oddball task in Fig. 4, comparing CSD waveforms and topographies to their surface potential counterparts for two commonly-used EEG reference schemes. At its typical peak latency (100 ms) for surface potentials, N1 is larger for a linked-mastoids compared to a nose reference at the depicted midline and lateral sites (first perpendicular orange line at 100 ms in Fig. 4A), while the corresponding topographies (Fig. 4B, top row) verify that the difference is due to the shift in the zero point from nose to mastoids. The CSD topography indicates that the origin of this shift is the proximity of the mastoids, but not the nose, to the sources that characterize N1. Moreover, volume-conduction renders a midline FCz maximum for both surface potential topographies (i.e., over an area without underlying cortical tissue), whereas the bilateral N1 sink-source patterns correctly identify two separate dipoles spanning the Sylvian fissure over each hemisphere, with sink maxima over mid-lateral sites (C3/4).
