A final concern that warrants consideration is the geometry of the skull and brain case model. A local Hjorth Laplacian does not imply a particular head shape, but rather a piecewise-planar surface. In contrast, spline interpolations are invariably limited by their capacity to adequately match the curvature of the skull and scalp (Carvalhaes and Suppes, 2011). With these concerns in mind, Babiloni et al. (1995) examined the performance of a various surface Laplacian estimates, suggesting that the choice of a measure could be matched to the particular clinical or research need. They also identified systematic errors for high-resolution montages (> 64 channels) that were related to the anatomical imprecision of the head model. A related concern is that topographic CSD features may be misrepresented, or even missed altogether, if inadequately sampled (e.g., using an extremely low-density montage, or one for which the spatial density grossly changes across the sampled scalp surface; cf. Tucker, 1993). Likewise, some localized generators may be difficult or impossible to identify if they vary across subjects with respect to bony landmarks (unless supported by imaging methods).
