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.