Event-related potentials (ERPs) are tiny electrical impulses from the brain that are elicited by many types of stimuli, such as tones, visually-presented words, and pictures.  The ERPs to a single stimulus are generally lost among all the other electrical activity the brain generates.  In order to see the ERPs clearly, many segments of  raw brain activity, each associated in time with the same stimulus type, are averaged together.  In this way, brain activity unrelated to the stimuli (noise) will be reduced and the ERPs will be more clearly seen.  The greater the number of segments that are included in the average, the clearer the ERPs will be.

Example

A subject is presented with 100 tones at the rate of one every second over earphones. Eighty of them are high pitched and 20 are low pitched. The high and low tones are randomly intermixed. The subject is instructed to press a button only to the low tones. While the subject is doing this task, his brain's electrical activity is recorded from sensors placed on the scalp. These are embedded in an elastic cap, as can be seen in the photograph on the right.

Two ERP averages can then be generated, one for the high and one for the low tones.  Some of the brain activity may not be recorded because it was contaminated with artifact (such as  muscle potentials from blinks).

Figure 2 shows the ERP data from one electrode (Cz, located at the center  of the top of the head) for the low tones and the high tones.  The up-arrow on the time line indicates when the stimulus was presented.  The size of the brain activity is indicated on the left in millionths of a volt or microvolts, and time is indicated on the bottom in milliseconds (1000 milliseconds = 1 second). The electrical activity labeled P3 is larger for the low than for the high tones.  This is a classic ERP response to rare stimuli and was first described here at the New York State Psychiatric Institute by the late Dr. Samuel Sutton.