The P300, first described by Sutton, Braren, Zubin, & John (1965), is one of the components of the brain's response to specific events that can be recorded from the scalp. These "event-related potentials" (ERPs) are manifestations of brain activities invoked in the course of information processing. The P300 reaches its maximal amplitude of at least 300 ms following a rare task-relevant stimuli. It is the largest at the parietal electrodes, somewhat smaller at the central electrodes, and minimal at the frontal electrodes.
Certain conditions are necessary for a given task to elicit a P300. First, a random sequence of stimulus events must be presented. Second, a classification rule that separates these events into two categories must be applied. Third, the task must require using the rule. Fourth, one category of events must be presented infrequently (Donchin, & Coles, 1988). As the P300 is elicited by events belonging to the rare category, its latency varies with the time required for categorizing the events. The amplitude of the P300 varies with the subjective probability and the task relevance of the eliciting events. Thus, the rarer the event, the larger the P300 it elicits.
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