Figure 1.

Peakmax coupling is broadband, and it begins after loss of consciousness on posterior electrodes before frontal electrodes. (a) Quantification of cross-frequency coupling. Two example traces from a frontal electrode (middle, “x”): “troughmax” (left) and “peakmax” (right). For each, the broadband EEG voltage (black) is bandpass filtered into low-frequency activity (LFA, orange) and high frequency activity ($\alpha$, light green). The instantaneous amplitude of the high frequency signal is computed using the analytic signal (${\alpha }_{\mathit{AMP}}$, dark green). The cross-frequency coupling is quantified by the correlation between the low frequency activity and the instantaneous amplitude of the high frequency signal. On the left the correlation is negative (troughmax, blue), and on the right the correlation is positive (peakmax, red). (b) Summary of cross-frequency coupling for one subject (28 years old). Top Panel: the target effect site concentration of propofol administered over the course of the session. Vertical lines indicate Loss of Consciousness (LOC) and Return of Consciousness (ROC). Second Panel: the probability of response curves for the two types of auditory stimuli (Verbal and Clicks). LOC was defined when the probability of response fell below 5% and remained for at least 5 min, and ROC was defined when the probability of response went above 5% and remained there for at least 5 min. Third and Fourth Panels: the cross-frequency coupling between the LFA and a range of amplitude frequencies (y-axis), for every 30 s interval in the session, for a set of five frontal electrodes (third panel) and a set of six posterior electrodes (fourth panel) with scalp positions indicated on the right. Fifth Panel: Scalp distribution of the cross-frequency coupling between the LFA and the 8–16 Hz amplitude.