Mach Bands

Mach bands is an optical illusion named after the physicist Ernst Mach, in which exaggerated contrast appears at the edges between two regions of slightly different gray shades the moment those regions touch, an effect sometimes also called the Chevreul illusion. The illusion arises through lateral inhibition within the retina, a process that makes the darker side of a boundary look even darker and the lighter side look even lighter right at the edge, effectively performing a kind of spatial high-boost filtering on the raw pattern of light striking the eye, describable mathematically as convolution with certain bandpass filters. Mach first reported the phenomenon in 1865, proposing that this filtering process happens within the retina's own neurons rather than later in the brain, a mechanism the illusion still shares with edge-detection processes throughout the visual system.