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Related to Mach bands: Simultaneous contrast
Illusory bands of intensified lightness and darkness perceived adjacent to the borders of light and dark in a visual image, caused by image processing in the retina and optic nerve.
When a light area is separated from a dark area by a transition zone in which the brightness increases or decreases regularly and rapidly, two bands are seen: one light band next to the dark area and another dark band next to the light area. The appearance of these two bands, known as Mach bands, is attributed to lateral inhibition processes occurring in the retina. The phenomenon is usually demonstrated with a rotating disc with black and white areas separated by a zone of brightness gradient. Syn. Mach rings.