Stare at the red dot on this
woman's nose for thirty seconds. Then, look over at a blank white space
(a piece of paper or an empty browser tab will do). Did you see the
"correct" version of the image? Here's how it works: stare long enough
at an object and the eye's photoreceptors (particularly the
color-sensitive cone cells) lose sensitivity from overstimulation.
Divert the eyes to a blank space, and the surrounding cone cells send
out a much stronger signal. The brain interprets this discrepancy as
looking at the precise opposite colors, in what is known as a 'negative
afterimage.' Nifty, no?