It turns out that one can mimic any perceived color by adjusting the ratio of stimulation of the three cone classes. This is how color televisions and monitors work. Each tiny pixel consists of three glowing phosphors: 1 red, 1 green and 1 blue. The relative intensity of these are altered to create any possible ratio of stimulation (excitiation) of each of the three cone classes.
The fact that almost any color can be created by adjusting the relative intensity of 3 primary colors is illustrated at the following web page:
If you take pure color and add equal amounts of the other two colors, the original color becomes more whitish: for example red turns to pink. The amount of white in a color is known as the amount of DESATURATION. A saturated color is one that appears very pure, while a desaturated color appears to have a lot of white in it.
Not all human beings have cones with exactly the same spectral absorption functions. Normal humans see color slightly differently from one another because there are small differences in L and M spectral absorption. More severe deviations from normality are referred to as color blindness. The most common form of color blindness is known as red-green color blindness. This may occur when the L and M cone have absorption functions which are very similar. In severe cases there is no L cone. An individual with only two cone types is knows as a dichromate or a deuteranope. Color blindness is discussed in detail on the following web site.
Dichromats cannot distinguish shades of green, yellow and red.
Most red-green colorblindness comes from a mutation on the X chromosome. Females have two X chromosomes, males only one. That is why it is much more common in males. The gene is fairly rare and recessive. A female has to inherit two copies of the same gene to be colorblind.
How is color information carried to brain? A small subset of retinal ganglion cells carry this information to brain. This is done in an interesting pattern.
which is illustrated at the following web site at at York.
Recall experiments with that showed ganglion
cell receptive fields. Ganglion cells tend to fire action potentials or nerve
spikes at a regular rate. When a light stimulus falls within the visual field
of the ganglion cell there is a change in the rate of nerve spikes.
There are two types of ganglion cells which carry color information to the
brain. RED/GREEN opponent cells respond to the difference between red and
green light within the receptive field. One color is excitatory the other
is inhibitory. They have inputs from L and M photoreceptors. BLUE/YELLOW opponent
cells respond to the difference in the amount of blue and yellow light in
their receptive field. They receive input from L+M cones (additive response
from two different cones), and input from S cones. These inputs are illustrated
in the web site above.
Ganglion cells which respond to brightness (e.g. the center-surround cells discussed earlier) receive their input from L cones and M cones, and the excitation is added. These ganglion cells respond to overall stimulus intensity, but do not send information about color.
Recall the case of lightness constancy -- objects appear about the same lightness under vastly different illumination. That is because the brain compares objects locally and assumes that there is an average illumination on all of them.
The same sort of thing happens with color. Objects do not change color appearance drastically with big changes in lighting -- even though the light reflected off objects will be totally dependent on the lighting.
This science is rather poorly understood. As with brightness contrast, however it is apparent that the brain notes the reflection off a variety of objects and averages out what is constant -- the illumination, and interprets colors as though the constant average coloration comes from the lighting -- not the objects themselves.
We can get some sense of what is going on by altering the color of the background of some colors.
The following website illustrates that our perception of color of objects is influenced by the average color of other objects and the background.
1. Explain how a person who is severely red-green color blind differs from a normal person.
2. How do retinal ganglion cells encode color and send it to the brain?
3. What is meant by the term "saturation" of a color?
Last Modified:
Tuesday, 04-Nov-2003 fleishml@union.edu