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FEATURE ARTICLE

How the Retina Works

Much of the construction of an image takes place in the retina itself through the use of specialized neural circuits

Helga Kolb

Retina Design According to Lifestyle

Figure 2. Diagram of a human eye . . .Click to Enlarge Image

All vertebrate retinas contain at least two types of photoreceptors—the familiar rods and cones. Rods are generally used for low-light vision and cones for daylight, bright-colored vision. The variations among animal eyes reveal adaptations to the different environments in which they live. Most fish, frog, turtle and bird retinas have three to five types of cones and consequently very good color vision. Keep in mind that reptiles and fish are "cold blooded" and need to be active in the warm daytime. Most mammals have retinas in which rods predominate. When the number of mammals started to explode as the dinosaurs died out, the Earth was likely a dark place covered in ash and clouds; the tiny, fur-covered early mammals were able to generate their own body heat and developed visual systems sensitive to dim light. Modern rodents such as rats and mice, which are nocturnal animals, still have retinas overwhelmingly dominated by rods; their cones are small in size and only make up 3 to 5 percent of their photoreceptors.

Figure 4. Cone photoreceptors . . .Click to Enlarge Image Figure 3. Cells in the retina . . .Click to Enlarge Image

Most other mammalian retinas also have a preponderance of rods, and the cones are often concentrated in specialized regions. In species such as cats and dogs, images focus to a central specialized area, aptly called the area centralis, where cones predominate. The retinas of mammals such as rabbits and squirrels, as well as those of nonmammals like turtles, have a long, horizontal strip of specialized cells called a visual streak, which can detect the fast movement of predators. Primates as well as some birds have front-projecting eyes allowing binocular vision and thus depth perception; their eyes are specialized for good daylight vision and are able to discriminate color and fine details. Primates and raptors, like eagles and hawks, have a fovea, a tremendously cone-rich spot devoid of rods where images focus.

Primates, in fact, have what is called a duplex retina, allowing good visual discrimination in all lighting conditions. The fovea contains most of the cones, packed together as tightly as physically possible, and allows good daylight vision. More peripheral parts of the retina can detect the slightest glimmer of photons at night. Most mammals have two types of cones, green-sensitive and blue-sensitive, but primates have three types—red-sensitive as well as the other two. With our cone vision, we can see from gray dawn to the dazzling conditions of high noon with the sun burning down on white sand. Initially the cone photoreceptors themselves can adapt to the surrounding brightness, and circuitry through the retina can further modulate the eye's response. Similarly, the rod photoreceptors and the neural circuitry to which they connect can adapt to lower and lower intensity of light.








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