A number of other reservations have repeatedly been cited in addition to the above concerns, including difficulties of computing the surface Laplacian at the edge of the EEG montage, the required explicit knowledge about the recording locations, and the need to quantify the surface Laplacian estimates. While the loss of information at border locations may be a sacrifice for finite difference methods (e.g., Hjorth, 1975), surface Laplacian estimates are readily available for these and at any other location by the use of spherical splines and other continuous interpolations (e.g., Nunez and Srinivasan, 2006; Pascual-Marqui et al., 1988; Perrin et al., 1989; reviewed by Carvalhaes and de Barros, 2015).
