P300

The P300 is an event-related potential (ERP) component — a positive voltage deflection in EEG brain signals that peaks approximately 300 milliseconds after a person perceives a rare or task-relevant stimulus among frequent non-target stimuli. It is named for its polarity (positive) and approximate latency (300ms). The P300 is involuntary, meaning it occurs automatically when a person recognizes a target stimulus, making it particularly valuable for brain-computer interfaces (BCIs) designed for people with severe motor disabilities such as locked-in syndrome. BCI typing systems like the P300 Speller and RSVP Keyboard detect this brain response to determine which letter or symbol a user intends to select, enabling text entry without any physical movement.