Opponent Process Theory

A theory for color vision first proposed by Ewald Hering in 1892, who disagreed with the prevailing theory at that: Trichromatic Theory ( Young-Helmholtz Theory ). Trichromatic theory suggests that color vision is based on three primary colors: red, green, and blue. Instead, Hering believed that human color vision is based on a system of three Opponent Channels, each comprising two opposing colors:

Where red, yellow, green and blue are the four Unique Hues, which are special in that any other color can be described as a mix of them, and that they exist in opposite pairs. Each channel is then an ‘antagonistic’ tug-of-war between its two Unique Hues. Hering’s main evidence for this theory was the observation of Afterimages.

Though the trichromatic and opponent processes theories were initially thought to be at odds, in 1925, Erwin Schrödinger (of cat fame) came to resolve the two theories as being almost perfectly compatible. As we know now, the 4th Unique Hue, yellow, exists not as its own photoreceptor, but as a synthesis of the red and green photoreceptors. Trichromatic Theory helps to explain how each type of Cone detects different Wavelengths of light. On the other hand, the Opponent Process Theory explains the downstream Phototransduction Pathway that determines how we actually perceive a color in our brain.

A schematic that demonstrates how Photoreceptor excitation (LMS Color Space) is transformed into Opponent Channels.

Both Opponent Process Theory and Trichromatic Theory are crucial in explaining the effects of colorblindness, so much so that the names of the opponent channels are taken as the primary types of colorblindness (red-green and blue-yellow).