Averaged CSD amplitude spectra (299 frequency points = variables) were submitted to unrestricted, covariance-based fPCA (29), with 67 electrodes, 2 conditions, and 82 participants (i.e., 10988 cases), followed by Varimax rotation of the covariance loadings (49; also see 50, 51). CSD-fPCA yields meaningful, physiologically identifiable spectral components that conform to the underlying data, while isolating and removing artifact (e.g., electromyogram and electrooculogram activity extracted as distinct components), thereby reducing noise and eliminating reference-related errors. Six factors accounted for 95% of the variance of amplitude spectra (cf. Section S1 in the Supplement), three of which (48% total variance) had clear peaks in alpha (low-alpha/theta, high-alpha and residual alpha; cf. 29), and had posterior maxima for eyes-closed.2
