Seeing Colors

Man, whose color perception evolved as a refinement of seeing, likewise developed an intelligence which made it less vital for him to distinguish colors. But the ability to see colors is still the product of harsh necessity for some animals.

So concludes Eye-man Gordon Lynn Walls (of Bausch & Lomb) in the current Journal of Applied Physics. Dr. Walls's theories will hardly quiet the old argument as to whether the bull sees red, or merely the movement of the matador's cape. Dog lovers will continue to protest the thought that their pets live in a colorless grey world.* But Biologist Walls outlines a hypothesis of color vision new to the layman. The ability to see colors Dr. Walls links directly to visual acuity--the ability to see well. He points out that the vertebrates with the greatest color vision (bony fishes, reptiles and birds, monkeys, apes and man) are those with the greatest visual acuity--and those most active in the daytime. "It is no accident," says he, that diurnality and hue-discrimination are associated, for they have a common basis in the structure of the retina."

Cones & Rods. The retina (the screen upon which the lens of the eye casts the image) has two kinds of visual cells: cones, each with its direct line to the brain; rods connected in multiple to the optic nerve fibers. The cones give sharp, color vision, work in bright light only. The rods "gang up" faint and dim impressions in weak light, catch no color. Some animals have cones but apparently no color vision; no known color-seeing animals have rod cells alone.

Sharp vision and color vision have two other common denominators. One is a highly sensitive dimplelike spot on the retina (the fovea centralis--literally, "central pit"), which acts as a magnifying device to spread the image over a greater number of visual cells. In this dimple, common to the vertebrates with the highest acuity (some birds have two in each eye), there are no rod cells. The cones are slim and tight-packed. The other common denominator is the mechanism for accommodation--ability to focus the eye, maintain a sharp image of a moving object.

Only mammals known to have color vision are the monkeys, apes and man. And they are the only mammals fully equipped with cones, dimples, and accommodation mechanisms. While man uses color vision largely for pleasure, comments Dr. Walls, it was first developed "by animals to whom magazine covers . . mean absolutely nothing."

* Trainers of the famed Seeing Eye Dogs take canine color blindness for granted. The dogs' apparent ability to distinguish traffic signals is really due to the dogs' alertness to traffic sounds and movement.

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