Understanding Psychology (10th Ed)

feature detection
The activation of 
neurons in the cortex by visual stimuli 
of specifi c shapes or patterns.
Optic tract 
Optic chiasm 
Optic nerve 
Right 
visual field 
Left visual field 
Primary visual area 
of cerebral cortex 
FIGURE 5 
Because the optic nerve 
coming from the eye splits at the optic 
chiasm, the image to a person’s right 
eye is sent to the left side of the brain 
and the image to the person’s left is 
transmitted to the right side of the 
brain. (
Source: Mader, 2000.)
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110 Chapter 
4 
Sensation and Perception
Color Vision and Color Blindness: 
The 7-Million-Color Spectrum
Although the range of wavelengths to which humans are sensitive is relatively nar-
row, at least in comparison with the entire electromagnetic spectrum, the portion to 
which we are capable of responding allows us great fl exibility in sensing the world. 
Nowhere is this clearer than in terms of the number of colors we can discern. A 
person with normal color vision is capable of distinguishing no less than 7 million 
different colors (Bruce, Green, & Georgeson, 1997; Rabin, 2004). 
Although the variety of colors that people are generally able to distinguish is 
vast, there are certain individuals whose ability to perceive color is quite limited—the 
color-blind. Interestingly, the condition of these individuals has provided some of the 
most important clues to understanding how color vision operates (Bonnardel, 2006; 
Neitz, Neitz, & Kainz, 1996). 
Approximately 7% of men and .4% of women are color-blind. For most people 
with color-blindness, the world looks quite dull (see Figure 7). Red fi re engines 
appear yellow, green grass seems yellow, and the three colors of a traffi c light all