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An event-related potential (ERP) in the brain is used to investigate the electrophysiological responses measured from the scalp by the electroencephalography (EEG) as a response to a certain event. This event is usually the exposition of a stimulus. As the EEG reflects thousands of simultaneously ongoing brain processes, the brain response to a certain stimulus or event of interest is usually not visible in the EEG. For this reason, the ERP is calculated by averaging the EEG data of dozens to thousands of stimulus expositions. Certain conditions manipulated by the independent variables (IV) of the stimulus set are compared, e.g., the presentation of a word and a nonword compared can reflect the temporal characteristics of semantic processing in the brain.
The most robust feature of the ERP response is a positive displacement (conventionally plotted downwards) occuring around 300ms after stimulus known as the P300 (or just P3). It represents conscious processing and its amplitude is greater with unpredictable stimuli. The paradigm of presenting mainly predictable stimuli mixed with occasional unpredictable stimuli is called "oddball". Earlier features of the ERP response depend on unconscious processing, and response also varies with electrode position on the scalp.
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