EEG processing
A 4-channel Sedline brain function monitor (Masimo, Irvine, CA, USA) was used for forehead EEG acquisition. The electrodes for the sensors record EEG between Fp1, Fp2, F7 and F8 with the ground electrode at Fpz, and the reference electrode at roughly 1 cm more than Fpz. A sampling rate of 178 Hz (16 bits) and a preamplifier bandwidth of 0.5–92 Hz was used to record the EEG data. In order to check the exact time and anesthesia level, an experienced researcher manually browsed EEG data of all of the patients. For the purpose of carrying out the spectral analysis, 10 s [