CHAPTER 15 The Special Senses

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Diseases and Disorders

TABLE 15.2

Vision

wavelength, thus allowing color perception over a wide range.Color is interpreted in the visual cortex of the occipital lobe ascombinations of sensory input originating from cone cells. Forexample, when orange light strikes the retina, 99% of the red-sensitive cones respond, 42% of the green-sensitive conesrespond, and no blue cones respond. When yellow light strikesthe retina, the response shifts, so that a greater number ofgreen-sensitive cones respond. The variety of combinations cre-ated allows humans to distinguish among several million grada-tions of light and shades of color.Not everyone sees the same red. Two forms of the red photo-pigment are common in humans. Approximately 60% of peoplehave the amino acid serine in position 180 of the red opsin protein,whereas 40% have alanine in that position. That subtle differencein the protein results in slightly different absorption characteristics(figure 15.22). Even though we were each taught to recognize redwhen we see a certain color, apparently we do not all see that colorin quite the same way.

Distribution of Rods and Cones in the Retina

Rods and cones are not evenly distributed across the retina. Conesare more numerous in areas of the retina associated with visualacuity. The macula and especially the fovea centralis functionwhen visual acuity is required, such as for focusing on the wordsof this page. The fovea centralis has about 35,000 cones and norods. The rest of the macula has more cones than rods. However,the 120 million rods are 10–20 times more plentiful than conesover most of the remaining retina. Rods are more highly concen-trated away from the macula and are more important in low-lightconditions.

Predict 5

Explain why at night a person may notice a movement “out of the cornerof the eye” but, when the person tries to focus on the area of movement,nothing seems to be there.