In marked contrast to an intracranial CSD, a surface Laplacian computed from a scalp montage clearly does not reflect neuronal generators within that surface (i.e., the scalp), but rather the currents that impinge on the scalp radially from the brain (e.g., Perrin et al., 1989; Giard et al., 1990). This implies that the surface Laplacian provides a coarse, but noninvasive, image of the subdural generators that have an effect on the montage from below, a simplification supported by the observed similarity of surface Laplacian waveforms and direct (invasive) recordings of field potentials at the dural surface (Junghöfer et al., 1997; Nunez et al., 1994), or by spatial deconvolution to that predicted at the dural surface (e.g., deblurring; Le and Gevins, 1993). The surface Laplacian may thereby be considered as a conservative description of essential constraints required of any proposed generator, in that even a precisely localized generator inferred from an inverse model is not plausible unless it is consistent with a Laplacian topography (cf. discussions in Tenke et al., 2010). Thus, a Laplacian topography reveals the spatial topography of underlying
