Fig. 4From: Time-trend analysis of the center frequency of the intrinsic mode function from the Hilbert–Huang transform of electroencephalography during general anesthesia: a retrospective observational studyInstantaneous frequencies during induction, maintenance, and emergence phases. The EEG signals of 30 patients were analyzed, and the instantaneous frequencies (HHT_IF) of each IMF were calculated. Each dot and error bar indicate the median and interquartile range of the HHT_IF. The region shaded in light blue indicates the frequency range of 8–12 Hz (alpha oscillations). *: p < 0.05; **: p < 0.01; ***: p < 0.001Back to article page