P300 (Neuroscience)

The P300 is a neural evoked potential component of the  electroencephalogram (EEG). This event-related potential (ERP) appears as a positive deflection of the EEG voltage at approximately 300  ms. It dominates at parietal electrode sites. The P300 is supposed to follow unexpected sensory stimuli or stimuli that provide useful information to the subjects according to his/her task.

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, 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 the useful property 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.

History
Since the P300 is an extremely robust event-related potential, it is possible to see it without sophisticated analysis methods or devices. It was discovered early in the research of event-related potentials in 1965 by Sutton and colleagues. Since then, an enormous amount of research has been done to study the nature of this deflection.

Applications
This is one of the most well-studied of the human EEG evoked components; it is thought to have applications in diagnostics or the construction of a viable brain-computer interface. It has also been studied for its possible use in screening for terrorists and other criminals, by seeing if they recognize items that an innocent person should not be able to do so.