Mach bands

   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. As a consequence, Mach bands are commonly classified as *entoptic phenomena. Because their occurrence is bound up with the inherent properties of the perceptual system, they can also be classified as "physiological illusions. Mach bands should not be confused with a related contrast illusion called the "Cornsweet effect.

   References

   Lotto, R.B., Williams, S.M., Purves, D. (1999). Mach bands as empirically derived associations. Proceedings of the National Academy of Sciences of the United States of America, 96, 5245-5250.

   Mach, E. (1865). Über die Wirkung der räumlichen Verteilung des Lichtreizes auf die Netzhaut. Sitzungsberichte der mathematischnaturwissenschaftlichen Classe der kaiserlichen Akademie der Wissenschaften, 52, 303-322.