The occipital lobes are at the back of the brain (see Figure 1)
When parts of the occipital lobes are electrically stimulated, people generally see colors or flashes of light. This area is referred to as the primary visual cortex because it is important for the processing of basic visual information, such as form, color, and movement. If the visual cortex is damaged, the patient typically develops problems perceiving these characteristics in all or a part of the visual field — the totality of what can be seen when a person looks straight ahead, which can be divided into the left visual field (everything seen on the left side) and the right visual field (everything seen on the right side). Oliver Sacks (1995) described the case of a 65-year-old artist who was unable to see colors after a car accident had damaged his visual cortex. He saw the world entirely in shades of black, white, and gray:
It was not just that colors were missing, but that what he did see had a distasteful, “dirty” look, the whites glaring, yet discolored and off-white, the blacks cavernous — everything wrong, unnatural, stained, and impure. … He saw people’s flesh, his wife’s flesh, his own flesh, as an abhorrent grey; “flesh-colored” now appeared “rat-colored” to him. … The “wrongness” of everything was disturbing, even disgusting, and applied to every circumstance of daily life. He found foods disgusting due to their greyish, dead appearance and had to close his eyes to eat. … His own brown dog looked so strange to him now that he even considered getting a dalmation. (pp. 7-8)
Reading a case study such as this, we begin to see how important color perception is for us in our everyday lives. In fact, compared to other animals, humans are among the best perceivers of color: we have large areas of our visual cortex devoted to this ability. It is interesting that the damage to this man’s visual cortex also was associated with an increase in his visual acuity — that is, the sharpness of his vision. As he put it, “I can see a worm wriggling a block away” (Sacks, 1995, p. 3).
Information from each side of the visual field crosses over and activates the opposite side of the primary visual cortex (see Figure 2). In other words, the primary visual cortex in the left hemisphere of the brain is activated by stimuli in the right visual field, whereas the primary visual cortex in the right hemisphere is activated by stimuli in the left visual field. Light from objects in the right visual field travels to the left side of the retina (the structure in the back of the eyes that contains visual receptors) in each eye, and the pathways from the left side of the retinas to the brain go only to the left hemisphere. The opposite is the case for light from objects in the left visual field: it goes to the right side of the retina in each eye, and the pathways from the right side of the retinas to the brain go only to the right hemisphere. The reason why visual information crosses over in this way is unknown; and, as you will see below, the same thing is true for other kinds of sensory information, as well as for motor messages that result in the movement of the skeletal muscles.
Study Questions for Section 3-7
- Where is the primary visual cortex located?
- What kinds of information are processed by the primary visual cortex?
- Which hemisphere of the primary visual cortex is activated by stimuli in the left visual field?
- Which hemisphere of the primary visual cortex is activated by stimuli in the right visual field?
- If the occipital lobes were destroyed, what would be the most likely result?
Quiz Questions for Section 3-7
Quiz Answers for Section 3-7
References
Sacks, O. (1995). An anthropologist on Mars: Seven paradoxical tales. New York: Knopf.
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