Seeing colour
Trichromacy and the opponent process
Two nineteenth century theories of colour vision that seemed to disagree. Both turned out to be right, at different stages.
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Two theories explain colour vision. The trichromatic theory says the eye has three kinds of receptor and builds every colour from their signals. The opponent-process theory says we see colour in opposing pairs: red against green, blue against yellow, light against dark. For decades they were rivals. Both are correct. The first describes the cones, the second what the nerve cells do next.
Three receptors
In 1802 the English physician Thomas Young proposed that the eye could not possibly have a separate receptor for every colour, and suggested three. The German scientist Hermann von Helmholtz developed the idea in the 1850s, and the Scottish physicist James Clerk Maxwell showed that almost any colour can be matched by mixing three lights. This explains why screens need only red, green and blue. The three cone types were finally measured directly in the twentieth century. Trichromacy simply means seeing with three channels of colour.
Opposing pairs
Ewald Hering, a German physiologist, pointed to things the three-receptor idea did not explain. People describe yellow as a pure colour, not as a mix of red and green. Nobody ever sees a reddish green or a yellowish blue. Stare at red and you get a green afterimage. In the 1870s Hering proposed three opposing pairs instead. In the 1950s the American researchers Leo Hurvich and Dorothea Jameson measured the pairs by cancelling one hue with its opposite, and nerve cells behaving in just this way were then found in the visual pathway.
How they fit together
The modern account has two stages. First, three kinds of cone catch the light. Then cells in the retina add and subtract the cone signals. One channel sets long-wavelength cones against medium ones, which is roughly red against green. Another sets short-wavelength cones against the other two combined, roughly blue against yellow. A third adds signals together for light and dark. The match with Hering's pure colours is close but not exact, and where and how the pure hues arise in the brain is still being worked out.
Sources and further reading
- Opponent process on Wikipedia
- Lee, The evolution of concepts of color vision (2008)
- Miyahara, Focal Colors and Unique Hues (2003)
- Nobel Prize in Physiology or Medicine 1967, for discoveries on the processes of vision
Original text by the Rang desk, written with AI assistance and checked against the sources above. Meanings of colour vary from place to place and person to person. If we have something wrong, tell us.