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An event-related potential (ERP) is any stereotyped electrophysiological response to an internal or external stimulus. More simply, it is any measured brain response that is directly the result of a thought or perception.
ERPs can be reliably measured using electroencephalography (EEG), a procedure that measures electrical activity of the brain through the skull and scalp. 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. One of the most robust features of the ERP response is a response to unpredictable stimuli. This response-known as the P300 (or simply "P3")-manifests as a positive deflection in voltage approximately 300 milliseconds after the stimulus is presented.
In actual recording situations, it is difficult to see an ERP after the presentation of a single stimulus. Rather the most robust ERPs are seen after many dozens or hundreds of individual presentations are averaged together. This technique cancels out noise in the data allowing only the voltage response to the stimulus to stand out clearly.
While evoked potentials reflect the processing of the physical stimulus, event-related potentials are caused by the "higher" processes, that might involve memory, expectation, attention, changes in the mental state etc.
Physicians and neurologists will sometimes use a flashing visual checkerboard stimulus to test for any damage or trauma in the visual system. In a healthy person, this stimulus will elicit a strong response over the primary visual cortex located in the occipital lobe in the back of the brain.
Experimental psychologists and neuroscientists have discovered many different stimuli, such as erotica (in a Washington University study), to elicit reliable EEG ERPs from participants. The timing of these responses is thought to provide a measure of the timing of the brain's communication or time of information processing. For example, in the checkerboard paradigm described above, in healthy participants the response of the visual cortex is around 150-200ms. This would seem to indicate that this is the amount of time it takes for the transduced visual stimulus to reach the cortex after light first enters the eye. Alternatively, the P300 response occurs at around 300ms regardless of the stimulus presented: visual, tactile, auditory, etc. Because of this general invariance in regard to stimulus type, this ERP likely reflects a higher cognitive response to new stimuli.
The P300 only peaks in the vicinity of 300 msec for very simple decisions. More generally, its latency appears to reflect the amount of time necessary to come to a decision about the stimulus. The harder the decision, the longer it takes for the P300 to appear. The leading theory, the context updating hypothesis (Donchin and Coles, 1988), is that it reflects an updating of expectancies about how probable events are in the current context. Because this updating cannot be conducted until the stimulus has been categorized, its latency is dependent on how long it took to come to the decision. One of its useful properties is that, unlike measure of physical responses like button pressing, the P300 appears to reflect only this stimulus evaluation time and not the time required to translate the decision into the physical response (such as which finger to use).
The P300 also has useful properties of being larger to rare stimuli, especially if they are targets. The amplitude of the P300 therefore gives information about how the person is categorizing the stimuli and how rare they are considered to be subjectively. The P300 is only seen when the person is actively keeping track of the stimulus so it also gives information about what they are paying attention to, which makes it useful for BCI applications. It also has proven to be quite sensitive to a wide variety of pathologies, usually being diminished in amplitude.
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