Fig. 1. For each period, the upper row shows the traces obtained at Cz (vertex) from each subject. Positivity is upward. The lower row shows the grand-averaged traces. Column A , labeled “no filter,” shows the raw traces. Column B , labeled “0–20 Hz,” shows waveforms after application of a 0- to 20-Hz digital filter to reveal the N1 and sustained potential (SP). Column C , labeled “30–50 Hz,” shows waveforms after application of a 30- to 50-Hz digital filter to reveal the 40-Hz auditory steady state response (40-Hz ASSR). The horizontal arrows indicate the timing of the stimulus train. Base = baseline; prop = propofol; reco = recovery.