Figure 9.
Surface Laplacian estimates [µV/cm2] employing optimized smoothing for different spherical spline orders (m = 2–5). The optimal value for λ was separately determined for each spline flexibility by computing the cross-validation (CV) criterion from the individual auditory N1 topographies (N = 164) at 112 ms constituting the overall grand mean 72-channel ERP topography (Fig. 8A). Accordingly, the sum of the squared differences between the observed potential and the estimated potential (i.e., using a 71-channel spherical spline interpolation with m flexibility and λ smoothing) was repeatedly computed for each of the 164 topographies at any site for different λ values to determine the minimum of the underlying function corresponding to a particular value of m (cf. Pascual-Marqui et al., 1988; Stone, 1974).