Figure 1 :The averaging method. In the left portion of the figure, the “raw” EEG (recorded from midline frontal, ventral and parietal channels, Fz, Cz and Pz) are traced and also the activity from two channels monitoring horizontal and vertical eye movement (hEOG and vEOG) activity is shown. A calibration signal displaying the size of the EEG and EOG deflections is provided. In this and all other figures, a positive deflection at an EEG scalp location relative to the reference electrode (the tip of the nose in this figure) is indicated by an upward deflection. The time calibration signal is provided in the x-axis. The EEG “epoch” or “sweep time” is 16 s. An auditory stimulus was presented to this subject while he/she read a book. The hEOG channel indicates that the subject did comply with the instructions as the eyes slowly moved from left to right, associated with the reading task. The vertical tick marks on each of the channels and on the x-axis indicates when the stimulus was presented. It is however not possible to observe the tiny change in the activity of the raw EEG. The ERP is thus embedded in the raw, ongoing EEG. Averaging is necessary to reduce the amplitude of the background EEG, allowing the ERP to emerge. The right portion of the figure shows the result of averaging a 500-ms “sweep” of the EEG following 200 stimulus presentations. The amplitude of this ERP is much reduced compared to the amplitude of the raw EEG. Note that the voltage scale is now 1 ěV. The sweep begins 50 ms prior to stimulus presentation and continues for 450 ms following it. The average of this pre-stimulus activity serves as a zero voltage baseline from which all peak deflections are measured. A large negative-going component of the auditory ERP is visible at about 100 ms. This is the N1. The entire procedure was repeated a second time. The two averages are then superimposed to provide an indication of the replicability of the results.