Figure 2 :The grand average (average of all subjects’ averages) auditory ERP following presentation of either a 60, 80 or 100 dB SPL brief duration stimulus. A stimulus was presented every 2 s. The stimuli were presented while the subject was awake and reading a book (thus ignoring the stimuli) and during NREM (mainly stage 2) and REM sleep. The ERPs recorded from the vertex (Cz) are displayed. The upward, closed arrow points to N1, peaking at about 100 ms. The open downward arrow points to P2 occurring at about 180–200 ms. In wakefulness, both N1 and P2 are gradually reduced in amplitude as the intensity of the stimulus is reduced. When the very obtrusive 100 dB stimulus is presented, a later positivity, peaking at about 280 ms was elicited. This is probably a P3a, perhaps reflecting a switch of attention away from the reading task and toward the auditory channel. During NREM sleep, N1 is at baseline level regardless of the intensity of the stimulus. On the other hand, P2 is larger in amplitude compared to the waking state. It is still however influenced by the intensity of the stimulus. During REM sleep, a small-amplitude N1 is visible followed by a P2. Adapted from Cote et al.