Von Kries Color Vision Theory

duplicity/duplexity theory. The duplicity/du-plexity theory of vision was first proposed by Max Schultz in 1866, and later by H. Parinaud and the German physiologist Johannes von Kries (1853-1928). The theory states that vision is mediated by two ("duplex") classes of retinal receptors, the cones that are "chromatic" and sensitive to color wavelengths and used in high illumination ("photopic vision"), and the rods that are "achromatic" and used in low illumination ("scotopic vision"). Because the two classes of receptors manifest different wavelength relationships, the shape of a specific function that relates brightness to color may be used to indicate whether rod or cone vision is predominant in a given situation [cf., the Anglo-American psychologist William McDougall's (1871-1938) early color vision theory that states that there are two distinct receptor mechanisms for light in the retina: rods for dim light and cones for normal and intense light; the theory holds, also, that all colors are reducible to the three basic colors of blues, greens, and reds]. There are anatomical differences between the rods and cones, even though these two types of receptors are very similar: (1) the rods are smaller and seem to be less highly developed than the cones; (2) there are no rods (only closely packed cones) in the foveal area of the retina; (3) the cones have a better ("one-to-one") supply of nerves;

(4) the substance rhodopsin ("visual purple") is present in the rods but not in the cones; and

(5) nocturnal animals possess mostly rods and very few cones. Today, the von Kries duplicity theory of vision is so well established that it counts as a strong statement of fact. See also COLOR VISION, THEORIES/LAWS OF; FOVEAL CONE HYPOTHESIS; von KRI-ES' COEFFICIENT LAW. REFERENCES

Schultze, M. (1866). Zur anatomie und physiologie der retina. Archiv der Mikroskopische Anatomisch, 2, 175286.

Kries, J. von (1895). Uber die natur gewisser mit den psychischen vorgangen ver-knupfter gehirnzustande. Zeitschrift furPsychologie, 8, 1-33. Parinaud, H. (1898). La vision. Paris: Octave Doin.

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