P300 is an
event-related potential that occurs upon
recognition of a given
stimulus in a series being unlike the previous stimuli, referred to as an
oddball stimulus in the literature. They are important in the study of
attention, as they occur when the attended part of
reality changes, replacing
boredom (or at least
inhibited response) with
interest in the new stimulus. P300 waves have been understood in
electrophysiology to mean that the subject is able to consciously
identify and
categorize a stimulus, and represent the subject updating his
working memory with the new information. For instance, if a subject has been listening to
trombone noises and a
flute tone is played, a P300 wave will appear 300 ms later on the
EEG machine.
Amplitude of the measured P300 wave is inversely related to the probability of the oddball stimulus. That is, the less frequent the oddball is, the more visible the P300 spike will be. Interestingly, a small P300 will appear for both categories of stimulus when they are presented at nearly the same frequency, and will be slightly larger for the slightly less frequent category. For instance, when asked to press one button if the presented letter is a vowel, and another if it's a consonant, there will be a P300 wave for both, and it will be higher for whichever letter type is less common.
P300 waves are present in people with most varieties of mental retardation, suggesting that their working memory is being updated in the same way as everybody else's. Psychotropic drugs, however, do have an effect on P300 -- Prozac attenuates it somewhat with even a single dose, and does so dramatically with repeated administration, though this hasn't been shown to have any clinical ramifications.