Understanding Psychology (10th Ed)

108 Chapter 
4 
Sensation and Perception
absence of nerve cells does not interfere with vision because you automatically com-
pensate for the missing part of your fi eld of vision. (To fi nd your blind spot, see 
Figure 4.) 
Once beyond the eye itself, the neural impulses relating to the image move 
through the optic nerve. As the optic nerve leaves the eyeball, its path does not take 
the most direct route to the part of the brain right behind the eye. Instead, the optic 
nerves from each eye meet at a point roughly between the two eyes—called the optic 
chiasm (pronounced KI-asm)—where each optic nerve then splits. 
When the optic nerves split, the nerve impulses coming from the right half of 
each retina are sent to the right side of the brain, and the impulses arriving from the 
left half of each retina are sent to the left side of the brain. Because the image on the 
retinas is reversed and upside down, however, those images coming from the right 
half of each retina actually originated in the fi eld of vision to the person’s left, and 
the images coming from the left half of each retina originated in the fi eld of vision 
to the person’s right (see Figure 5). 

PROCESSING THE VISUAL MESSAGE

By the time a visual message reaches the brain, it has passed through several stages 

of processing. One of the initial sites is the ganglion cells. Each ganglion cell gathers 
information from a group of rods and cones in a particular area of the eye and com-
pares the amount of light entering the center of that area with the amount of light 
in the area around it. Some ganglion cells are activated by light in the center (and 
darkness in the surrounding area). Other ganglion cells are activated when there is 
darkness in the center and light in the surrounding areas. The outcome of this process 
is to maximize the detection of variations in light and darkness. The image that is 
passed on to the brain, then, is an enhanced version of the actual visual stimulus 
outside the body (Kubovy, Epstein, & Gepshtein, 2003; Pearson & Clifford, 2005; 
Lascaratos, Ji, & Wood, 2007). 
The ultimate processing of visual images takes place in the visual cortex of the 
brain, and it is here that the most complex kinds of processing occur. Psychologists 
David Hubel and Torsten Wiesel won the Nobel Prize in 1981 for their discovery that 
many neurons in the cortex are extraordinarily specialized, being activated only by