P300 (P3)
The P300 is a positive bump in the EEG that shows up roughly 300 milliseconds after something rare, meaningful, or surprising catches your attention. It is the brain's involuntary "oh, that one!" reaction — you do not have to decide to make it happen; it just fires when a stimulus you care about stands out from the ordinary ones around it.
Because the bump is tied to what you are paying attention to rather than to any muscle movement, it gives a BCI a clean handle on a person's focus. A system flashes many options and watches for the telltale 300 ms wave; whichever option triggered it must be the one you were waiting for. This is the engine behind P300 spellers, which let people type just by attending to a letter.
In a classic "oddball" task, you watch a stream of mostly identical tones and silently count the rare odd one out. Each rare tone produces a clear positive deflection in the EEG that peaks around 300 ms after it sounds, while the frequent common tones do not.
The oddball task: a rare, attended stimulus evokes the ~300 ms positive wave.
The exact peak is not always at 300 ms — it tends to come later when the task is harder or the person is tired, and it shifts with age — so the "300" is a rough label, not a fixed clock reading. It is one member of a larger family of event-related potentials.