event-related potential (ERP)

An event-related potential is a small, stereotyped ripple in the EEG that is locked in time to a specific event — a sound, a flash, a decision. Its shape is consistent enough that scientists give its peaks names by their direction and timing, like N100 or P300 ("N" for negative, "P" for positive, the number for the rough delay in milliseconds).

The catch is that this response is tiny — far smaller than the brain's constant background chatter, so a single trial is buried in noise. The trick is repetition: present the same kind of event many times and average all the snippets together. The random noise cancels itself out, while the true, time-locked response survives and stands out clearly.

ERPs are the backbone of several BCI paradigms, because each one is a reliable little stamp the brain leaves whenever a meaningful event occurs. The P300 used in spellers is the most famous example of an ERP put to work for control.