“The concept that no two people see exactly the same thing when actively using their eyes in a natural situation is shocking to some people because it implies that not all men relate to the world around them in the same way. . . .
How there can be such great differences in the visual worlds of two people becomes clearer if it is known that the retina (the light-sensitive part of the eye) is composed of at least three different parts or areas: the fovea, the macula, and the region where peripheral vision occurs. Each area performs different visual functions, enabling man to see in three very different ways. Because the three different types of vision are simultaneous and blend into each other, they are not normally differentiated. The fovea is a small circular pit in the center of the retina containing roughly 25,000 closely packed color-sensitive cones, each with its own nerve fiber. The fovea contains cells at the unbelievable concentration of 160,000 cells per square millimeter (an area the size of the head of a pin). The fovea enables the average person to see most sharply a small circle ranging in size from 1/96 of an inch to 1/4 of an inch (estimates differ) at a distance of twelve inches from the eye. The fovea, also found in birds and the anthropoid apes, is a recent development in evolution. . . . In man, needle-threading, removal of splinters, and engraving are some of the many activities made possible by foveal vision. Without it there would be no machine tools, microscopes, or telescopes. . . .
Surrounding the fovea is the macula, an oval, yellow body of color-sensitive cells. It covers a visual angle of 3 degrees in the vertical plane and 12 to 15 degrees in the horizontal plane. Macular vision is quite clear, but not as clear and sharp as foveal vision because the cells are not as closely packed as they are in the fovea. Among other things man uses the macula for reading.
The man who detects movement out of the corner of his eye is seeing peripherally. Moving away from the central portion of the retina, the character and quality of vision change radically. The ability to see color diminishes as the color-sensitive cones become more scattered. Fine vision associated with closely packed receptor cells . . . shifts to very coarse vision in which perception of movement is enhanced. . . . Peripheral vision is expressed in terms of an angle, approximately 90 degrees, on each side of a line extending through the middle of the skull. . . . Thus even though man sees less than a one-degree circle sharply, the eyes move so rapidly as they dart around painting in the details of the visual world that one is left with the impression of a much wider clear area than is actually present in the visual field. The fact that attention is focused on foveal and macular vision in co-ordinated shifts also maintains the illusion of broad-band clear vision.”
—Edward T. Hall, from The Hidden Dimension, 1966.