ABSTRACT
The P300 (also known as the P3) is a class of EVENT-RELATED POTENTIALS (ERP) of positive polarity, with a peak latency which can vary anywhere from 300 to 600 or more msec, and which is elicited by relatively rare or salient stimulus events in any sensory modality. Among the most common of the experimental procedures employed to elicit the P300 is the ODDBALL TASK in which relatively rare stimuli must be discriminated from more frequently occurring background stimuli. In such tasks, the amplitude of the P300 is inversely proportional to the probability of the eliciting stimulus and its peak latency co-varies with the time required to classify the stimulus. These properties, along with the fact that it can as well be elicited by the OMISSION of an expected stimulus as by the presentation of an unexpected one, have led to the P300 being regarded as a truly ENDOGENOUS component, reflecting the engagement of high-level cognitive processes. The P300 does not represent the activity of only one brain region. Experimental and neuropsychological studies indicate that the component reflects the activity of multiple regions, including inferior PARIETAL CORTEX and DORSOLATERAL PREFRONTAL CORTEX, and that the relative contributions of these regions varies according to stimulus and task factors. The cognitive functions indexed by the P300 have been much debated, but remain uncertain. Because of its relatively large size and reliability, and the simplicity of the procedures required to elicit it, the P300 has received much interest as a possible measure of cognitive impairment in studies of neurological and psychiatric illness. This interest was fuelled by the finding that the peak latency of the component lengthens with age, and is even more prolonged in individuals with dementing illnesses such as ALZHEIMER's DEMENTIA. The variation in the P300 measures within the population is however too great to permit them to be used reliably as an aid to diagnosis. While the P300 is undoubtedly a valuable marker of cognitive function in the context of group studies, uncertainty about its neural origins, and the cognitive functions it reflects, limit its usefulness.
