Their sense of security or danger continued to depend

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Unformatted text preview: ned to function without optical support. Their sense of security or danger continued to depend upon the visual cues that give them their perception of depth. The rat, in contrast, does not depend predominantly upon visual cues. Its nocturnal habits lead it to seek food largely by smell, when moving about in the dark, it responds to tactual cues from the stiff whiskers (vibrissae) on its snout. Hooded rats tested on the visual cliff show little preference for the shallow side so long as they can feel the glass with their vibrissae. Placed upon the glass over the deep side, they move about normally. But when we raise the center board several inches, so that the glass is out of reach of their whiskers, they evince good visual depthdiscrimination: 95 to 100 per cent of them descend on the shallow side. Cats, like rats, are nocturnal animals, sensitive to tactual cues from their vibrissae. But the cat, as a predator, must rely more strongly on its sight. Kittens proved to have excellent depth-discrimination. At four weeks– about the earliest age that a kitten can move about with any facility–they invariably choose the shallow side of the cliff. On the glass over the deep side, they either freeze or circle aimlessly backward until they reach the center board. The animals that showed the poorest performance in our series were the turtles. The late Robert M. Yerkes of Harvard University found in 1904 that aquatic turtles have somewhat poorer depthdiscrimination than land turtles. On the visual cliff one might expect an aquatic turtle to respond to the reflections from the glass as it might to water and so prefer the deep side. They showed no such preference: 76 per cent of the aquatic turtles crawled off the board on the shallow side. The relatively large minority that choose the deep side suggests either that this turtle has poorer depth-discrimination than other animals, or that its natural habitat gives it less occasion to "fear" a fall. All of these observations square with what is known about the life his...
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This document was uploaded on 01/30/2014.

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