Unlike the resting EEG, which is a recording of ongoing brain activity, ERPs reflect brain electrical activity in response to specific sensory or cognitive events occurring at a specific time. ERPs can be used to monitor brain activity ranging from sensory reception to higher cognitive processes. ERPs are characteristic, highly reproducible waveforms that are displayed graphically (figure 2A) as a series of peaks (designated P for positive components) and valleys (designated N for negative components). Components are described in terms of their wave height (amplitude, measured in microvolts [μV]), and the time of their occurrence following presentation of the stimulus (latency, measured in milliseconds [ms]). For example, P300 is a positive peak occurring around 300 ms after the stimulus; it also is designated as P3 (i.e., third positive peak). Early components, those with a latency of less than 100 ms, reflect responses to the physical characteristics of the stimulus, whereas later components are influenced by more cognitive factors.
