Figure 6 :The processing of a long duration stimulus. A 1.3-s auditory stimulus was presented to four subjects while they were awake and after they fell into stage 2 sleep. The grand averaged ERP consisted of a transient N1 (initial closed arrow) and P2-on at the onset and a smaller N1 (second closed arrow) and P2-off at the offset. A sustained potential (SP) is apparent throughout the duration of the stimulus. The SP is maximum in amplitude over frontal sites. During stage 2 sleep, an N1 was near baseline level but a large P2 and N350 (open arrow) were elicited. The apparent slow potential may represent an SP or simply the slow return of the N350 to baseline.