Hockenbury Discovering Psychology 5th txtbk

Vision95nearsightedness, or myopia, distant objects appear blurry because the light reflectedoff the objects focuses in front of the retina. In farsightedness, or hyperopia, objectsnear the eyes appear blurry because light reflected off the objects is focused behindthe retina. During middle age, another form of farsightedness often occurs, calledpresbyopia. Presbyopia is caused when the lens becomes brittle and inflexible. Inastigmatism, an abnormally curved eyeball results in blurry vision for lines in a particulardirection. Corrective glasses remedy these conditions by intercepting andbending the light so that the image falls properly on the retina. New surgical techniques,such as LASIK, correct visual disorders by reshaping the cornea so that lightrays focus more directly on the retina.The RetinaRods and ConesThe retina is a thin, light-sensitivemembrane that lies at the back of theeye, covering most of its inner surface(see Figure 3.3). Contained inthe retina are the rods and cones.Because these sensory receptor cellsrespond to light, they are often calledphotoreceptors. When exposed tolight, the rods and cones undergo achemical reaction that results in aneural signal.Rods and cones differ in manyways. First, as their names imply, rodsand cones are shaped differently. Rods are long and thin, with blunt ends. Conesare shorter and fatter, with one end that tapers to a point. The eye contains far morerods than cones. It is estimated that each eye contains about 7 million cones andabout 125 million rods!Rods and cones are also specialized for different visual functions. Although bothare light receptors, rods are much more sensitive to light than are cones. Once therods are fully adapted to the dark, they are about a thousand times better than conesat detecting weak visual stimuli (Masland, 2001). We therefore rely primarily onrods for our vision in dim light and at night.Rods and cones also react differently to changes in the amount of light. Rodsadapt relatively slowly, reaching maximum sensitivity to light in about 30 minutes.In contrast, cones adapt quickly to bright light, reaching maximum sensitivity inabout 5 minutes. That’s why it takes several minutes for your eyes to adapt to thedim light of a darkened room but only a few moments to adapt to the brightnesswhen you switch on the lights.You may have noticed that it is difficult or impossible to distinguish colors in verydim light. This difficulty occurs because only the cones are sensitive to the differentwavelengths that produce the sensation of color, and cones require much more lightthan rods do to function effectively. Cones are also specialized for seeing fine detailsand for vision in bright light.Most of the cones are concentrated in the fovea, which is a region in the verycenter of the retina. Cones are scattered throughout the rest of the retina, butthey become progressively less common toward the periphery of the retina. Thereare no rods in the fovea. Images that do not fall on the fovea tend to be perceivedas blurry or indistinct. For example, focus your eyes on the word For at thebeginning of this sentence. In contrast to the sharpness of the letters in For, thewords to the left and right are somewhat blurry. The image of the outlying wordsis striking the peripheral areas of the retina, where rods are more prevalent andthere are very few cones.Slim Rods and Fat ConesThe rods and cones in the retina arethe sensory receptors for vision. Theyconvert light into electrical impulsesthat are ultimately transmitted to thebrain. Color has been added to thisscanning electro micrograph to clearlydistinguish the rods and cones. Therods, colored green, are long, thin, andmore numerous than the cones, coloredblue, which are tapered at oneend and shorter and fatter than therods. As the photo shows, the rods andcones are densely packed in the retina,with many rods surrounding a fewcones.retina(RET-in-uh) A thin, light-sensitive membranelocated at the back of the eye that containsthe sensory receptors for vision.rodsThe long, thin, blunt sensory receptors ofthe eye that are highly sensitive to light, butnot to color, and that are primarily responsiblefor peripheral vision and night vision.conesThe short, thick, pointed sensory receptorsof the eye that detect color and are responsiblefor color vision and visual acuity.fovea(FO-vee-uh) A small area in the center of theretina, composed entirely of cones, wherevisual information is most sharply focused.