Trial–averaged evoked ERPs for each subject-electrode were decomposed into real-valued time-frequency energy surfaces E(t,f) to evaluate the evoked ERP’s intensity at specific times (t = 512, −2 to 2 seconds) and frequencies (f = 129, DC to 64 at .5 Hz steps). Also, each subject-electrode’s degree of phase-locking to the stimulus across trials was quantified by computing “phase-locking factor” (PLF, also called inter-trial phase coherence; Delorme and Makeig, 2004; Tallon-Baudry et al., 1996) from the complex-valued (containing phase-information) C(t,f) surface. For each set of N (number of trials) Cs extracted from each subject-electrode, i, we computed PLFi(t,f) as 1N∑j=1Nexp(ϕij(t,f)) where ϕij represents the energy-normalized phase-angles from C for that electrode on trial j. PLF ranges from zero, indicating randomness of phases across trials, to one, indicating perfect phase-locking across trials. Most commonly, time-frequency energy and PLF have been computed using wavelet transforms (Delorme and Makeig, 2004; Ford et al., 2008; Roach and Mathalon, 2008; Tallon-Baudry et al., 1996), but to circumvent characteristic time-smearing (low time-frequency resolution) associated with wavelets, we used reduced interference distribution (RID) transforms belonging to Cohen’s class
