In EEG-based BCIs, the P300 potential is defined as?

• A. The instantaneous peak of alpha rhythm during relaxed eyes-closed state.
• B. A brain signal observable about 300 ms after a perceived rare stimulus, often used in speller applications.
• C. A motor-related potential preceding limb movement.
• D. A resting-state baseline used for calibration.

Answer: (B)

The P300 is an event-related potential that appears as a positive deflection roughly 300 milliseconds after a rare, task-relevant stimulus that the person is actively attending to. In EEG-based BCIs, especially spellers, the user focuses on a target symbol that flashes among non-targets; the brain’s evaluation of that attended, infrequent stimulus produces the P300, and the system uses this signal to infer the user’s choice. It’s typically strongest over the centro-parietal regions and reflects cognitive processing of stimulus significance and attention, not simple sensory encoding or motor plans. This distinguishes it from the instantaneous alpha peak seen during relaxed eyes-closed states, motor-related potentials that precede movement, or resting-state baselines used for calibration.