Fig. 4.

(a) and (e) shows the covariance of the spectral amplitudes, during visual stimulation, calculated either by averaging the mean inter-trial covariance across participants (a) or looking at how the spectral estimates, averaged over trials, co-vary across participants, (e). In both cases the amplitude of the lower frequencies (0–30 Hz) appears correlated across frequencies, whereas the induced gamma response at 40–60 Hz appears less correlated, suggesting separate generative mechanisms. For each of these two analyses, panels b–d and f–h show the cross-correlation of the spectra for alpha, beta and gamma, either across trials (top row) or subjects (bottom row). Red dots demonstrate significant correlations (p < 0.05 corrected for multiple comparisons). Note how correlations are strongest within frequency bands. Panels f–h demonstrate the across-trials correlation after z-transforming and averaging over subjects for the same frequency bands. The shaded bars represent the standard deviation of the Z-score across subjects.