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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.
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