Each trial flashes a small square on the left or right side of the screen — sometimes so briefly you won't see anything. After each flash, two questions: first, whether you saw anything; second, which side it was on.
Answer the second question even when you answer no to the first. That's the point. Be honest on both — the experiment depends on it. 32 trials.
Keep your eyes on the cross in the center of the canvas. Don't let them drift.