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 flexible spline (smaller m) localizes it more precisely. However, for a 71-channel montage, the N1 sink topography is clearly overresolved for the most flexible spline (m = 2). With high-density montages, the precision of each electrode placement and the reproducibility and size of the electrode-scalp interface must also be considered (Tenke and Kayser, 2001; Greischar et al., 2004) before computing and interpreting a Laplacian based on this variability. With this in mind, a flexibility of m = 4 is frequently chosen for scalp ERPs (cf. Perrin et al., 1989), resulting in consistent CSD topographies for group averages using high- or low-density montages (Fig 6A; cf. Kayser and Tenke, 2006a,b). However, a flexibility of m = 3 is well-considered for application to motor processes with a stable, known localization (e.g., Burle et al., 2008). Likewise, a flexibility of m = 5 may stabilize the topography resulting from less consistent or variably localized processes (but see Carvalhaes and Suppes, 2011, for limitations).
