CPT — measures sustained attention and impulse control. About once a second a letter appears. Levels 1-5: tap EVERY X (classic X-CPT). Levels 6-15: AX-CPT — tap X ONLY if it followed an A (working-memory load); pace rises and X-look-alike letters are added. Duration is fixed at ~90 seconds — difficulty grows by TASK difficulty, not by time. Biomarkers: omission errors, commission errors, mean RT, RT variability (CV-RT — strong ADHD marker), vigilance decrement toward the end of the session.