Also known as Mach’s bands and Mach band illusion. All three eponyms refer to the Czech-born Austrian physicist and philosopher Ernst Mach (1838–1916), who first described the concomitant phenomenon in 1865. They refer to a contrast illusion that can be perceived in two or more adjacent fields with different hues, interconnected by a colour gradient. This contrast illusion consists of darker and brighter bands bordering the edges of the colour gradient, lending the edge of the light field an even lighter appearance and the edge of the dark field an even darker appearance. Although the mediation of Mach bands is not fully understood, an important role is attributed to reciprocal lateral inhibition by adjacent cones in the retina.