P300 speller

A P300 speller is a way to type using attention alone. Letters and symbols are laid out in a grid on a screen, and the rows and columns flash in a quick, shuffled order. The user just looks at — and silently attends to — the letter they want, doing nothing else.

Each time the row or column containing that target letter flashes, it counts as a rare, meaningful event, and the brain answers with a P300 bump. Because the flashes that crossed your chosen letter are the ones followed by that bump, the system can work backward from the timing of the P300 to figure out exactly which letter you meant.

It is slow compared with a keyboard, but it asks almost nothing of the body — no clicking, no speech, not even a steady gaze in many designs — which makes it valuable for people who have lost most voluntary movement.