Fig. 4. ( Bottom row ) The median spectrograms of the study groups R0, R1, and R2 calculated from the electroencephalogram during induction of anesthesia. The median spectrograms are calculated by choosing the group's median value at every time-frequency point. ( Top and middle rows ) The difference spectrograms calculated from the median spectrograms of two specific groups. The median spectrograms are determined from the time-normalized spectrograms and therefore given as a function of relative time r . The power in spectrograms is given in arbitrary units.