Le and Gevins (1993) arrived at a similar solution using a completely different approach. Eq. 2 was defined for scalp-recorded potentials, but based on a multiple shell model of the head, whereby the CSD generators of interest were posited to exist below the skull on the dural surface. Just as Nicholson identified Eq. 5 as a convolution integral by which a CSD source (or sink) imposes a field potential profile across the conductive medium, a dural source (sink) produces a field potential topography across the scalp. Given approximate thicknesses and impedances of the scalp and skull, the weighting function can be estimated and the scalp potential image deblurred (i.e., deconvolved) to regain the image at the dura. Nunez et al. (1994) reported that the image produced by the scalp Laplacian compared favorably with one produced by deblurring. Not surprisingly, Junghöfer et al. (1997) noted a relationship between deblurring methods, including CSD and cortical mapping, and the inverse problem. Another application of this approach has been proposed by Yao (2001) to yield an estimate for reference-free field potentials termed reference electrode
