Even though the surface Laplacian is identified with high-resolution EEG, CSD estimates using a reduced montage are often surprisingly stable, particularly for group averages (Kayser and Tenke, 2006b; Tenke et al., 2011). Fig. 6A shows auditory N1 ERP and corresponding CSD topographies from a 71-channel montage, as well as subsets as sparse as 16-channels. The overall topography of N1 is quite stable, as is the amplitude and location of the N1 sink. The corresponding source topography is also surprisingly preserved, even when it is poorly represented in the montage. However, the spatial resolution of a spline estimated CSD can also be directly varied. Fig. 6B illustrates the impact of spline flexibility on the spatial resolution of a common spherical spline Laplacian (Perrin et al., 1989; Kayser, 2009). All topographies show a common generator pattern consisting of a current sink anterior to the Sylvian fissure, coupled with a current source posterior to it. With a stiffer spline (i.e., larger m constant; cf. Eq. 2 in Kayser and Tenke, 2006a; Perrin et al., 1989), the sink is coarsely localized, while a more
