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2 to eliminate the potential distance cue provided by

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Unformatted text preview: rom a moving car appear to pass by more quickly than distant ones (Fig. 2). To eliminate the potential distance cue provided by pattern density we increased the size and spacing of the pattern elements on the deep side in proportion to its distance from the eye (Fig. 3, top). With only the cue of motion parallax to guide them, adult rats still preferred the shallow side, though not so strongly as in the standard experiment. Infant rats chose the shallow side nearly 100 per cent of the time under both conditions, as did day-old chicks. Evidently both species can discriminate depth by differential motion alone, with no aid from texture density and probably little help from other cues. The perception of distance by binocular parallax, which doubtless plays an important part in human behavior, would not seem to have a significant role, for example, in the depth perception of chicks and rats. Two types of visual depth-cue are diagrammed schematically on this page. Ellipses approximate the visual field of an animal standing near the edge of the cliff and looking toward it; diagrams at right give the geometrical explanation of differences in the fields. The spacing of the pattern elements (solid color) decreases sharply beyond the edge of the cliff (top). The optical motion (shaded color) of the elements as the animal moves forward (center) or sideways (bottom) shows a similar drop-off. To eliminate the cue of motion parallax we placed the patterned material directly against the glass on either side of the board but used smaller and more densely spaced pattern-elements on the cliff side. Both young and adult hooded rats preferred the side with the larger pattern, which evidently "signified" a nearer surface. Day-old chicks, however, showed no preference for the larger pattern. It may be that learning plays some part in the preference exhibited by the rats, since the young rats were tested at a somewhat older age than the chicks. This supposition is supported by the results of our experiments with animals reared in the dark. The effects of early experience and of such deprivations as dark-rearing represent imp...
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