[Ewald Hering's opponent colors. History of an idea]

Ewald Hering (1834-1918) was one of the founders of modern visual science. Among his contributions, his color concept gave rise to mostly fruitless controversies, because it appeared to be incompatible with the trivariance of vision theory put forward by Young and Helmholtz. It is now clear that the two color concepts can be reconciled with one another. Hering's theory is based upon an analysis of visual perception. Experience indicates that a conscious subject needs four unique colors in order to characterize perception: blue, yellow, red and green. Usually, two of these hues, but never more, constitute a color sensation, e.g., orange contains red and yellow while blue and green are the components of turquoise. The central idea in Hering's concept was that red and green are opposite hues because they are never elicited simultaneously by a color stimulus; the same is true for blue and yellow. Hering also postulated that the perception of opponent colors is mediated by opponent processes in certain elements of the nervous system, i.e., that there are neurons that respond in a qualitatively different way to spectral stimuli of different frequencies. Today we know that, in fact, the majority of neurons in the retina and the visual pathway is capable of opponent-type responses as anticipated by Hering. It was, however, a long way until his concept was understood and finally accepted by the science community. Among those who helped to acknowledge it were the German physiologist von Kries, the Austrian physicist Schrödinger, the Finnish neurophysiologist Svaetichin, and the American psychophysicists Hurvich and Jameson. In 1906, the German Ophthalmological Society honored Ewald Hering by awarding him the Albrecht von Graefe Medal.