Ewald Hering (full name Karl Ewald Konstantin Hering) (August 5, 1834 - January 26, 1918) was a German physiologist who did much research into color vision and spatial perception. Hering disagreed with the leading theory developed mostly by Thomas Young and Hermann von Helmholtz. Helmholtz's theory stated that the human eye perceived all colors in terms of three primary colors: red, green, and blue. Hering instead believed that the visual system worked based on a system of color opponency, a proposal now widely recognized as correct.
Hering looked more at qualitative aspects of color and said there were six primary colors, coupled in three pairs: red-green, yellow-blue and white-black. Any receptor that was turned off by one of these colors, was excited by its coupled color. This results in six different receptors. It also explained afterimages. His theory was rehabilitated in the 1970s when Edwin Land developed the Retinex theory that stated that whereas Helmholtz's colors hold for the eye, in the brain the three colors are translated into six.
In 1861 Hering described an optical illusion which now bears his name - Hering illusion. He also developed Hering's law of equal innervation to describe the conjugacy of saccades in animals.
References
R. Steven Turner, In the eye's mind : vision and the Helmholtz-Hering controversy (1994, Princeton University Press).
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