Asahi Illusion

The Asahi illusion, named with the Japanese word for morning sun, is an optical illusion in which a flower-shaped figure with petals arranged in a circle, very dark on their outer edges and growing steadily lighter toward the center, causes viewers to perceive the central area as significantly brighter than it actually is. Researchers from the University of Helsinki and the Max Planck Institute in Tubingen, including Nelson Totah, discovered the illusion while studying the brains and pupils of rats, and found that the gradient pattern triggers the pupillary reflex, the mechanism by which the brain constricts the pupils in response to bright light to protect the retina, even though the perceived brightness driving that constriction is illusory. Because the illusion produced a measurable pupil response in rats as well as in humans, the researchers concluded that the link between visual perception and pupil size arose earlier in mammalian evolution than previously believed.