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Vision Opponent Process Theory

Explanations > Perception > Visual Perception > Vision Opponent Process Theory

Description | Discussion | So what?

Description

The sensors in the eye that detect color are called cones. Each cone sends back three signals to the brain, containing:

• Lightness, or how bright the color is.
• A process: Red-green, as a point on the spectrum between red and green.
• B process: Blue-yellow, as a point on the spectrum between blue and yellow.

The brain then combines these signals into the perception of all colors that we experience as normal vision. If the A process and B process shows no preference for red-or-green, or blue-or-yellow, then we see black and white.

Each cone/rod set effectively forms a kind of pixel, so our whole sighted image is a combination of all these 'dots' (it is not exactly like this, but pixels are a useful analogy).

This system works through a process of excitatory and inhibitory responses, where red, yellow and white excite 'opponent neurons' while green, blue and black inhibit them.

Given a set of three cones that each focus (approximately) on red, green and blue:

• Luminance comes from a combination of all three cone signals, plus nearby rod signals.
• A process comes from excitation of red and inhibition of green cone signals.
• B process combines red, green and blue with excitation of yellow frequencies and inhibition of blue.

Example

An opponent color chart looks like this:

Discussion

This theory was developed by Ewald Hering in 1920, in contradiction to the prevailing red, green and blue (RGB) trichromatic theory that was based on light (along with the pigment variant of red, blue and yellow).

An effect of this is that we perceived hues mixed from A plus B channels, the single channel signal means we cannot mix signals within this. Hence, while we can see blue and red together in a bluey-red hue, we do not see bluey-yellow, nor reddish-green.

The excitation and inhibition of opponent process also explains how, when we look at images, colors on one side of the vision opponent chart stand out and attract the eye more than colors on the other side. Hence:

• White seems to come forward while black recedes.
• Yellow seems to come forward while blue recedes.
• Red seems to come forward while green recedes.

It is not surprising that red and yellow are used a lot in brand imagery that seeks to be bold and brash. MacDonald's, for example. Red also is a symbol of danger and hence arouses the mind, ready for fight-or-flight.

In photography and painting, an image that has a lot of yellow and red can be overpowering, and photographers tend to use these with care. A 'touch of red' may be used deliberately to create an initial focus for the eye. In contrast, blues and greens are often seen in images that seek to create a sense of calm.

The fact that color blindness often means people have difficulty in distinguishing red and green suggests a problem with the A-process system.

There is a puzzle in Trichromatic theory, where red, green and blue light can be mixed to form all other hues, yet red, yellow and blue paint have a similar pigment effect. Why is only yellow and green changed? While we know the physics of direct and reflected light, it makes for a confusing model of light, particularly from the perceptive position. In opponent theory, all of these colors have equal weight.

Vision Opponent Process is also known as Natural Color System, LAB Color Space (or LaB or L*A*B or CIELAB). LAB stands for Luminance (black/white), A-process (red/green) and B-process (blue/yellow). LAB color processing is sometimes available in photo editors as a variant on the more common RGB process (that is more natural for computers).

Goethe's color wheel to some extent predates Hering's theory as it shows red and green on opposing sides, and then yellow and violet. It also has orange against blue and does not include black and white.

So what?

In designing visual images, use red, yellow and white to help attract attention to the most important parts of the picture. In particular use these to grab the eye to where it looks first. Beware of over-tiring the eye when doing this. Green, blue and black may be used as background hues or to create a mood of calm. For subtle effects in video, you can show one color for a while, then switch to white and allow viewers to see a ghostly reversed image for a moment.

See also

The Meaning of Colors

Hering, E. (1920/1964). Outlines of a Theory of the Light Sense. Translated by L.M. Hurvich and D. Jameson, of Grundzuge der Lehre vom Lichtsinne, Cambridge, MA: Harvard University Press.