Hering Illusion

The Hering illusion is one of the classic geometrical-optical illusions, discovered by the German physiologist Ewald Hering in 1861. Two straight, parallel lines are drawn over a radial background resembling bicycle spokes radiating from a central point, and although both lines are perfectly straight, they appear to bow outward, away from each other, because of the radiating pattern behind them. The illusion has two related variants, the Orbison illusion, which produces a similar effect, and the Wundt illusion, which produces a comparable but inverted distortion. Several competing explanations have been proposed for why it occurs, including Hering's own original idea that viewers overestimate the angles formed where the lines cross the radiating background, lateral inhibition within the visual cortex, theories involving how the brain converts a three-dimensional scene into a two-dimensional projection, and a more recent account holding that the radial pattern mimics the visual signature of forward motion, tricking the brain's motion-processing system, which normally compensates for processing delays by extrapolating where a moving object will be next, into bending the static straight lines as though they were moving.