The P300 is a positive-going waveform, occuring about 300 ms poststimulus, that is hypothesized to reflect memory updating, especially that dealing with language processing. Present in ERP recordings wherein the participant updates previously formed memories, it is said to reflect whether relatively short-term memory systems are intact.

The P300 is highly manipulatable: It is significantly affected, in terms of latency and amplitude, by the current arousal levels and biological cycles of the subject being tested.
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.

An event-related potential initially evoked by presenting a sequence of similar tones followed by an odd-ball tone. Interesting applications have been developed by a group of neurophysiologists in the American midwest.

Lie detection is traditionally done by measuring heart rate, skin conductance, and rate of breathing. These measures are sensitive to individual health issues. An example is the case of Los Alamos Laboratories, where federal investigators demanded that top nuclear physicists turn over lists of medication to polygraph operators.

The P300 event-related potential can be used to determine concealed knowledge that only a criminal would know. By placing details of the crime(s) randomly among a list of non-relevant items, one can distinguish criminal from citizen. If an individual recognizes a detail of the crime, they produce both a P300 event-related potential and are also guilty of, or at least familiar with, the crime.

A second application of P300 recognition waveforms is the diagnosis of Alzheimer's Disease. Dr. John Polich and several others have shown that as Alzheimer's progresses, P300 amplitude decreases and latency increases. While individual differences in neuromechanics prevent this from being a perfect diagnosis of the disease's progress, it can assist doctors in charting an individual's decline.

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