Saylor URL: http://www.saylor.org/books Saylor.org
the experience of color. According to this Young-Helmholtz trichromatic color theory, what
color we see depends on the mix of the signals from the three types of cones. If the brain is
receiving primarily red and blue signals, for instance, it will perceive purple; if it is receiving
primarily red and green signals it will perceive yellow; and if it is receiving messages from all
three types of cones it will perceive white.
The different functions of the three types of cones are apparent in people who
experience color blindness—the inability to detect either green and/or red colors. About 1 in 50
people, mostly men, lack functioning in the red- or green-sensitive cones, leaving them only able
to experience either one or two colors (Figure 4.15).
Figure 4.15
People with normal color vision can see the number 42 in the first image and the number 12 in the second (they are
vague but apparent). However, people who are color blind cannot see the numbers at all.
Source: Courtesy
ofhttp://commons.wikimedia.org/wiki/File:Ishihara_11.PNG andhttp://commons.wikimedia.org/wiki/File:Ishiha
ra_23.PNG.
The trichromatic color theory cannot explain all of human vision, however. For one, although the
color purple does appear to us as a mixing of red and blue, yellow does not appear to be a mix of