As noted above, montage density is another critical consideration for EEG data, affecting surface potential and surface Laplacian estimates alike. Dense electrode arrays (64 or more electrodes), affording whole head coverage with interelectrode distances below 2 cm, are generally preferred to a low-density EEG montage (less than 21) with interelectrode distances greater than 6 cm (e.g., Nunez and Srinivasan, 2006). The effect of reducing the spatial sampling from 72 to 31 scalp locations for an auditory N1 topography is shown in Figure 8B. The average reference, now based on fewer sites, is slightly altered, as can be inferred the isopotential contour lines. CSD estimates are also affected, resulting in the unsystematic removal of higher spatial frequencies from the signal (i.e., spatial integration). This effect is most noticeable for more flexible spline interpolations (correlations between 72- and 31-channels topographies across 80381 surface points, for m = 2, r = 0.5954; for m = 3, r = 0.8916), whereas less flexible splines largely retain the 72-channel topography (for m = 4, r = 0.9752; for m = 5, r = 0.9754). Thus,
