Event-Related Brain Potentials and Behavior I: Measurement, Motor Activity, Hemispheric Asymmetries, and Sleep
Another measure of brain activity, derived from EEG recordings, is the event-related brain potential (ERP). Unlike the EEG, which represents spontaneous brain activity, the ERP is generated as a response to specific stimuli, and is an average of a number of samples. These ERPs are time-locked to stimulus events and have proven valuable to the psychophysiologist interested in a record of brain responses to stimuli, even when no other noticeable response occurs. A great deal of research effort has been devoted to studying the relationship between ERPs and human psychological activities. Most of the work has been conducted over the past 35 years and is continuing at a high rate. What makes the ERP so appealing is the possibility of relating specific brain responses to discrete psychological states and events. The extensive research on ERPs indicates that it rivals heart activity as the most popular physiological variable studied by psychophysiologists.
The ERP has been found to be dependent on both physical and psychological characteristics of stimuli, although in some instances, ERPs are independent of specific stimuli. For example, brain responses have been found to occur at the precise time that stimuli were expected but not actually presented ( Sutton, Teuting, Zubin, & John, 1967). When ERPs occur independently of external stimuli, they are called endogenous, indicating that they are produced by internal events. Those ERPs produced as a reaction to specific external events are called exogenous potentials.
Vaughan ( 1969) proposed the term event-related potentials to refer to a variety of brain responses that show stable time relationships to actual or anticipated stimuli. Those ERPs were classified by Vaughan as (a) sensory ERPs, (b) motor potentials, (c) long-latency potentials, and (d) steady potential shifts (SPS).
The sensory ERPs include those produced by visual, auditory, somatosensory (e.g., touch), and olfactory stimuli. Examples of sensory ERPs are shown in Fig. 5.1. They are based on the composite averaged potentials of eight individuals. The various negative and positive waves (components) of these ERPs can be seen in the illustration. Motor potentials (MP) refer to potentials that precede and accompany voluntary movement. Vaughan observed that the amplitude of the MP varies with the strength and speed of muscle contraction.
The long-latency potentials refer to those positive or negative components of the ERP that occur at 250 to 750 msec after some event. They reflect subjective responses to expected or unexpected stimuli, including the orienting response (see Ritter, Vaughan, & Costa, 1968). This is especially true for a positive component occurring at about 300 msec after stimulus onset, termed P300, and also referred to as P3 (discovered by Sutton, Braren, & Zubin, 1965).