Understanding Psychology (10th Ed)

Neuroscience in Your Life:
Seeing Vision in the Brain
FIGURE 6 
In addition to having specialized neurons that respond more strongly to particular 
shapes, orientations, and widths, our brains process information coming from each eye 
separately, creating what are called ocular dominance columns. With new MRI techniques, 
researchers can note the activity of the each eye more precisely. In these scans we can see the 
variation in ocular dominance columns for three individuals. The red areas show the response 
of one eye, and blue areas show the response of the other. 
(Source: Shmuel et al., 2010.)
Participant 1
Participant 2
Participant 3
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Module 11 
Vision: Shedding Light on the Eye 
111
look yellow. In fact, in the most common form of color-blindness, all red and green 
objects are seen as yellow. In other forms of color-blindness, people are unable to tell 
the difference between yellow and blue. In the most extreme cases of color-blindness, 
which are quite rare, people perceive no color at all. To such individuals, the world 
looks something like the picture on an old black-and-white television set. 

EXPLAINING COLOR VISION

To understand why some people are color-blind, we need to consider the basics of color 

vision. Two processes are involved. The fi rst process is explained by the
 trichromatic 
theory of color vision ,
which was fi rst proposed by Thomas Young and extended by 
Hermann von Helmholtz in the fi rst half of the 1800s. This theory suggests that there 
are three kinds of cones in the retina, each of which responds primarily to a specifi c 
range of wavelengths. One is most responsive to blue-violet colors, one to green, and 
the third to yellow-red (Brown & Wald, 1964). According to trichromatic theory, per-
ception of color is infl uenced by the relative strength with which each of the three 
kinds of cones is activated. If we see a blue sky, the blue-violet cones are primarily 
triggered, and the others show less activity.
However, there are aspects of color vision that the trichromatic theory is less 
successful at explaining. For example, the theory does not explain what happens after 
you stare at something like the fl ag shown in Figure 8 on page 112 for about a min-
ute. Try this yourself and then look at a blank white page: You’ll see an image of the 
traditional red, white, and blue U.S. fl ag. Where there was yellow, you’ll see blue, 
and where there were green and black, you’ll see red and white. 
The phenomenon you have just experienced is called an afterimage . It occurs 
because activity in the retina continues even when you are no longer staring at the 
original picture. However, it also demonstrates that the trichromatic theory does not 
explain color vision completely. Why should the colors in the afterimage be different 
from those in the original?