The survival of a species requires that its members

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Unformatted text preview: tory and ecological niche of each of the animals tested. The survival of a species requires that its members develop discrimination of depth by the time they take up independent locomotion, whether at one day (the chick and the goat), three to four weeks (the rat and the cat) or six to 10 months (the human infant). That such a vital capacity does not depend on possibly fatal accidents of learning in the lives of individuals is consistent with evolutionary theory. To make sure that no hidden bias was concealed in the design of the visual cliff we conducted a number of control experiments. In one of them we eliminated reflections from the glass by lighting the patterned surfaces from below the glass (to accomplish this we dropped the pattern below the glass on both sides, but more on one side than on the other). The animals–hooded rats–still consistently chose the shallow side. As a test of the role of the patterned surface we replaced it on either side of the centerboard with a homogeneous gray surface. Confronted with this choice, the rats showed no preference for either the shallow or the deep side. We also eliminated the optical difference between the two sides of the board by placing the patterned surface directly against the undersurface of the glass on each side. The rats then descended without preference to either side. When we lowered the pattern 10 inches below the glass on each side, they stayed on the board. We set out next to determine which of two visual cues plays the decisive role in depth perception. To an eye above the center board the optical pattern on the two sides differs in at least two important respects. On the deep side distance decreases the size and spacing of the pattern elements projected on the retina. "Motion parallax," on the other hand, causes the pattern elements on the shallow side to move more rapidly across the field of vision when the animal moves its position on the board or moves its head, just as nearby objects seen f...
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