In 1870 Ewald Hering, a German physiologist and
psychologist proposed a new theoretical approach.
He postulated that certain color perception
processes take place not in the
photoreceptors
but at a later
stage
of the visual process,
as the data progresses towards the brain.
Hering maintained there are four psychological primary colors,
and that the visual system operates in pairs of opposed colors:
red-green and yellow-blue, each color complementing its counterpart .
An additional achromatic pair, black and white, is responsible for all
the shades of gray. As contrasting colors cannot merge,we cannot
conceive of, let alone see, a yellowish purple or reddish green.
It now became possible to explain certain visual phenomena which the trichromatic theory did not explain sufficiently well. For instance, color blind people cannot see red or green but can see yellow despite the fact that yellow is a secondary color produced by additive
color mixing
of green and red.
Another phenomenon that Hering's theory explained is color persistence. This occurs when for example we look at a yellow surface for a few minutes, and then look at a light gray surface. The eye retains the perception of color, a negative afterimage, in this case the color blue, which is complementary to yellow . Hering's theory also explains the phenomenon of simultaneous contrast, where juxtaposed opposed colors enhance each other.
The greens and reds of these vegetables enhance each other naturally.
The bow is visible on an orange background, but does not stand out.
On a contrasting color, purple, the bow comes alive, practically glowing.
Hering's theory has become known as the opponent-process theory. It postulates that opponent processes take place not in the
photoreceptors
but in a later
stage
of the visual process toward the brain. Modern research demonstrates that the Young-Helmolz theory does apply to the primary
stage
of
color vision
since there are three types of color receptors (cones): one type sensitive to blue, one to green, and the third to the yellow-red range. Color vision is thus a two-stage process: the
photoreceptors
respond differentially to three colors, these responses are encoded in the bipolar cells and in the optic nerve according to four-color system for
transmission
to the higher cerebral visual centers.
