Averaged ERP waveforms were transformed into current source density (CSD) estimates (μV/cm2 units) using a spherical spline surface Laplacian50 as detailed elsewhere (e.g.,48,53; for documented Matlab code, see 54). To determine common sources of variance in these reference-free transformations of the original ERP data, CSD waveforms were submitted to temporal principal components analysis (PCA) derived from the covariance matrix, followed by unrestricted Varimax rotation of the covariance loadings. However, only a limited number of meaningful, high-variance CSD factors were retained for further statistical analysis (for complete rationale, see 42,43,48,51). By virtue of the reference-independent Laplacian transform, CSD factors have an unambiguous component polarity and topography.
