Neurophysiological assessment
head movements
P300 recording was done for all included patients pre-operatively and one week postoperatively by an expert neurophysiologist who was blinded to the intraoperative drugs used. The used device was Galileo Series preamplifiers acquisition system (EBN, Florence, Italy). Bioelectrical activity of the brain was recorded by using gold electrodes and a conductive EEG paste (Elefix). Three active electrodes were positioned at the central line of the scalp frontally (Fz), centrally (Cz), and parietally (Pz) according to the international 10–20 system. While reference electrodes were placed on the earlobes, the ground electrode was positioned at the forehead. Impedance of all electrodes in all recordings was under 5 kΩ. The patients were instructed to keep their eyes closed and avoid head movements in order to reduce eye and muscle artifacts during P300 recording. All participants underwent auditory oddball paradigm task testing. They were instructed to listen to the series of tones with closed eyes. The auditory stimuli were pure tones presented binaurally at random intervals ranging between 3 and 4 s. All tones were 100 ms in duration with a rise–fall time of 10ms and were adjusted in intensity to a 70-dB sound pressure level. The pitches of frequent (standard) and infrequent (target) tones were 1000 and 3000 Hz, respectively, with a total number of 200 tones and a presentation probability for the infrequent tones of (0.2). The patients were asked to identify the infrequent tones in one task by raising the right index finger of the dominant hand in response to the infrequent tones as targets during 15-minute session.The ERP data were averaged with the sweep beginning 100 ms before the stimuli and lasting until 900 ms after stimulus onset in a common average montage containing 3 channels of midline electrode locations Fz, Cz, and Pz according to the international 10–20 system of EEG electrode placement. The P300 latency was identified as the largest positive peak at range of 250–700 ms occurring after the N1, P2, and N2 ERP components obtained from the ‘‘target’’ stimulus presentation. The peak amplitudes were evaluated as the differences between the P300 peak and the mean baseline automatically calculated by a computer [