Fading Disk

FADING DISK shows the effect of fatigue on your visual system. A screen contains a green background with a fuzzy-edged red spot in the middle. When you stare at the dot, it begins to fade. For many people, the dot fades completely away; for others a vague yellow smudge replaces it, and for others the background fades as well. The visual receptors in the eye need a changing stimulus. This is usually provided by sharp borders being moved by the jitter of the eyeball. Because this dot doesn't have sharp borders, the eye has nothing to focus on and the image fades.