Letters will appear one at a time. Press Space or the button only when the current letter matches the letter from 2 steps ago (e.g., A → B → A = Match!).
Each letter shows for 500ms, then a blank for 1500ms. Respond as quickly as you can when you see a match.
N-back keeps you updating a short rolling buffer while new stimuli arrive. The 2-back version is demanding but short — a common choice when you want a repeatable working-memory load without a 20-minute battery.