Another important notion concerns the different topographies across reference schemes. In contrast to ERP topographies, which do not change after converting to a different reference (i.e., the between-topography correlations depicted in Fig. 6A for either N1 or P3 are all r = 1.0), spectral topographies differ substantially between EEG references (for theta, 0.0548 ≤ r ≤ 0.3707; for alpha, 0.6895 ≤ r ≤ 0.8697), because spectral estimates are derived from nonlinear data transformations (cf. Fig. 1 in Tenke and Kayser, 2005). Thus, topographies of spectra and, by extension, of all oscillation measures based on power spectra, including phase locking, coherence, event-related spectral perturbations and many more, are affected by the choice of reference – CSD-based spectral measures are not (e.g., cf. Fein et al., 1988; Guevara et al., 2005; Schiff, 2005).
