The spherical spline interpolation of continuous surface potentials on the scalp surface, which is derived from the n discrete locations included in the EEG montage, involves a recurrence term to solve a Legendre differential equation of degree n (Perrin et al., 1989; cf. Eq. 3 in Kayser and Tenke, 2006a). To obtain a valid solution for this iterative series to yield a sufficient precision for creating a data transformation matrix, a minimum number of iterations are required. In general, a larger number of iterations will generate better results, however, as any improvements will become increasingly smaller with additional iterations, the computational costs will eventually outweigh their gain. Using a 19-channel EEG montage, Perrin et al. (1989) noted that a minimum of 7 iterations were required for a spline order of m = 4 to obtain a precision of 10−6. Because the precision level will be affected by the spline order and montage density, a minimum of 20 iterations, but preferably 50 or more, is a good choice. Importantly, the time-consuming process for setting-up two montage-dependent transformation matrices (one for estimating
