The”VisuaI Cliff”
This simple apparatus is used to in vestigate depth perception in different anitnals. All species thus far tested seem able to perceive and a void a sharp drop as soon as they can move about by Eleanor J. Gibson and Richard D. Walk H uman infants at the creeping and visual cliff [see photograph on cover}. the experiments with nonhuman infants. toddling stage are notoriously We tested 36 infants ranging in age On the visual cliff we have observed the prone to falls from more or less from six months to 14 months on the behavior of chicks, turtles, rats, lambs, high places. They must be kept from visual cliff. Each child was placed upon going over the brink by side panels on the center board, and his mother called kids, pigs, kittens and dogs. These ani mals showed various reactions, each of their cribs, gates on stairways and the him to her from the cliff side and the which proved to be characteristic of vigilance of adults. As their muscular shallow side successively. All of the 27 their species. In each case the reaction is coordination matures they begin to avoid infants who moved off the board crawled plainly related to the role of vision in such accidents on their own. Common out on the shallow side at least once; the survival of the species, and the varied sense might suggest that the child learns only three of them crept off the brink patterns of behavior suggest something to recognize falling-off places by experi onto the glass suspended above the pat about the role of vision in evolution. ence-that is, by falling and hurting him tern on the Boor. Many of the infants self. But is experience really the teacher? crawled away from the mother when she ception Or is the ability to perceive and avoid a called to them from the cliff side; others rapidity. At an age of less than 24 hours brink part of the child’s original endow cried when she stood there, because they the chick can be tested on the visual ment? could not come to her without crossing cliff. It never makes a “mistake” and al In the chick, for example, depth per manifests itself with special Answers to these questions will throw an apparent chasm. The experiment thus ways hops off the board on the shallow light on the genesis of space perception demonstrated that most human infants side. Without doubt this finding is re in general. Height perception is a special can discriminate depth as soon as they lated to the fact that the chick, unlike case of distance perception: information can crawl. many other young birds, must scratch in the light reaching the eye provides stimuli that can be utilized for the dis for itself a few hours after it is hatched. he behavior of the children in this Kids and lambs, like chicks, can be crimination both of depth and of reced T situation gave clear evidence of their tested on the visual cliff as soon as they can stand. The response of these animals ing distance on the level. At what stage dependence on vision. Often they would of development can an animal respond peer down through the glass on the deep is equally predictable. No goat or lamb effectively to these stimuli? Does the on side and then back away. Others would ever stepped onto the glass of the deep ,et of such response vary with animals of pat the glass with their hands, yet de side, even at one day of age. When one different species and habitats? spite this tactual assurance of solidity of these animals was placed upon the glass on the deep side, it displayed char At Cornell University we have been would refuse to cross. It was equally investigating these problems by means clear that their perception of depth acteristic stereotyped behavior. It would of a simple experimental setup that we had matured more rapidly than had refuse to put its feet down and would call a visual cliff. The cliff is a simulated their back up into a posture of defense, its one and hence makes it possible not only ported themselves on the glass over the front legs rigid and its hind legs limp. In to control the optical and other stimuli deep side as they maneuvered awk this state of immobility it could be locomotor abilities. Many sup (auditory and tactual, for instance) but wardly on the board; some even backed pushed forward across the glass until its also to protect the experimental subjects. out onto the glass as they started toward head and field of vision crossed the edge It consists of a board laid across a large the mother on the shallow side. Were it of the surrounding solid surface, where sheet of heavy glass which is supported not for the glass some of the children upon it would relax and spring forward a foot or more above the Boor. On one would have fallen off the board. Evident upon the surface. side of the board a sheet of patterned ly infants should not be left close to a At the Cornell Behavior Farm a group material is placed Bush against the un brink, no matter how well they may of experimenters has carried these exper dersurface of the glass, giving the glass discriminate depth. iments with kids and goats a step further. the appearance as well as the substance This experiment does not prove that the human infant’s perception They fixed the patterned material to a and sheet of plywood and were thus able to the same material is laid upon the Boor; avoidance of the cliff are innate. Such an adjust the “depth” of the deep side. this side of the board thus becomes the interpretation is supported, however, by With the pattern held immediately be- of solidity. On the other side a sheet of 64 © 1960 SCIENTIFIC AMERICAN, INC CHILD’S DEPTH PERCEPTION is tested on the visual cliff. The apparatus consists of a board laid across a sheet of heavy glass, with a patterned material directly beneath the glass on one side and several feet below it on the other. Placed on the center board (top left), the child crawls to its mother across the “shallow” side (top right). Called from the “deep” side, he pats the glass (bottom left), but despite this tactual evidence that the “cliff” is in fact a solid surface he refuses to cross over to the mother (bottom right) . 65 © 1960 SCIENTIFIC AMERICAN, INC function without optical support. by smell, when moving about in the neath the glass, the animal would move to about the glass freely. With the optical Their sense of security or danger con dark, it responds to tactual cues from the Hoor dropped more than a foot below the tinued to depend upon the visual· cues stiff whiskers (vibrissae) on its snout. glass, the animal would immediately that give them their perception of depth. Hooded rats tested on the visual cliff freeze into its defensive posture. Despite The rat, in contrast, does not depend show little preference for the shallow repeated experience of the tactual solid predominantly upon visual cues. Its noc side so long as they can feel the glass ity of the glass, the animals never learned turnal habits lead it to seek food largely with their vibrissae. Placed upon the KITTEN’S DEPTH PERCEPTION also manifests itself at an early age. Though the animal displays no alarm on the shallow side (top), (bot· tom) ; in some cases it will crawl aimlessly backward in a circle. it “freezes” when placed on the glass over the deep side 66 © 1960 SCIENTIFIC AMERICAN, INC glass over the deep side, they move about normallv. But when we raise the center board s�veral inches, so that the glass is out of reach of their whiskers, they evince good visual depth-discrimina tion: 95 to 100 per cent of them descend on the shallow side. ats, like rats, are nocturnal animals, C 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-discrimi nation. 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 [see illustratiol1s 011 oppo site page]. The animals that showed the poorest performance in our series were the tur tles. The late Robert M. Yerkes of Har vard University found in 1904 that aquatic turtles have somewhat poorer depth-discrimination than land turtles. On the visual cliff one might expect an aquatic turtle to respond to the reHec tions 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 rela tively 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 history and ecological niche of each of the animals tested. The survival of a species re quires that its members develop dis crimination of depth by the time they take up independent locomotion, wheth er 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 de pend on possibly fatal accidents of learn ing in the lives of individuals is con sistent 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 elimi nated reHections from the glass by light ing 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 consist ently chose the shallow side. As a test of the role of the patterned surface we GOATS SHOW DEPTH PERCEPTION at an age of only one day. A kid walks freely on the shallow side (top) ; on the deep side (middle) it leaps the “chasm” 10 safety (bo//,om). 67 © 1960 SCIENTIFIC AMERICAN, INC – – – – • – – – – – – • – – • – – – • – – – – • – \, – • • ., .. ——-;,. . – – • • — • – – t t >. • – : – • … .,. I .” ” ,” ,, I I I : I TWO TYPES OF VISUAL DEPTH·CUE are diagrammed sche· in the fields. The spacing of the pattern elements (solid color) matically on this page. Ellipses a p proximate the visual field of an decreases sharply heyond the edge of the cliff (tol1). The op· animal standing near the edge of tbe cliff and looking toward it; tical motion (shaded color) of the elements as the animal moves diagrams at right give the geometrical explanation of differences forward (center) or sideways (bottom) shows a similar drop·off. 68 © 1960 SCIENTIFIC AMERICAN, INC replaced it on either side of the cen terboard with a homogeneous grai sur face. Confronted with this choice, the rats showed no preference for either the shallow or the deep side. We also elimi nated 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 pref erence to either side. ‘vVhen we lowered the pattern 10 inches below the glass on each side, they stayed on the board. This supposition is supported by the re sults of our experiments with animals reared in the dark. The effects of early experience and of e set out next to determine which cisive 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 distance decreases the size and gators have pointed out, however, that the dark-reared rats required a certain amount of “pretraining’; in the light be fore they could be made to jump. Since the visual-cliff technique requires no pretraining, we employed it to test such deprivations as dark-rearing repre groups of light-reared and dark-reared of maturation and learning in animal be groups showed sent important clues to the relative roles havior. The first experiments along this line were performed by K. S. Lashley and James T. Russell at the Univer sity of Chicago in 1934. They tested W of two visual cues plays the de side rats, since the young rats were tested at a somewhat older age than the chicks. light-reared and dark-reared rats on a “jumping stand” from which they in duced animals to leap toward a platform placed at varying distances. Upon find ing that both groups of animals jumped with a force closely correlated with dis tance, they concluded that depth per ception in rats is innate. Other investi- spacing of the pattern elements pro hooded rats. At the age of 90 days both the same preference for the shallow side of the apparatus, confirming Lashley’s and Russell’s con clusion. Recalling our findings in the young rat, we then took up the question of whether the dark-reared rats relied upon motion parallax or upon contrast in tex ture density to discriminate depth. When the animals were confronted with the visual cliff, cued only by motion parallax, they preferred the shallow side, as had the light-reared animals. When the jected 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 from a moving car – – appear to pass by more quickly than dis tant ones [see illustration on opposite p age ]. To eliminate the potential dis tance 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 [see top illustration at right]. 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 d’Jpth by dif ferential motion alone, with no aid from texture density and probably little help from other cues. The perception of dis tance by binocular parallax, which doubtless plays an important part in 11U man behavior, would not seem to have a significant role, for example, in the depth perception of chicks and rats. To eliminate the cue of motion paral lax we placed the patterned material di rectly 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 eviden�ly “signified” a nearer surface. Day-old chicks, however, showed no preference for the larger pat ter.n. It may be that learning plays some part in the preference exhibited by the SEPARATION OF VISUAL CUES is shown in these diagrams. Pattern density is held constant (top) by using a larger pattern on the low side of the cliff; the drop in optical remains. Motion parallax is equalized (bottom) by placing motion (motion parallax) patterns at same level; the smaller pattern on one side preserves difference in spacing. 69 © 1960 SCIENTIFIC AMERICAN, INC ••• • • • • • • IMPORTANCE OF PATTERN in depth perception is shown in left), presumably because it “signified” a nearer and therefore these photographs. Of two patterns set at the same depth, normal safer surface. Confronted with two patternless surfaces set at dif. rats almost invariahly preferred the larger (top row and bottom ferent depths, the animals displayed no preference (bottom right). 70 © 1960 SCIENTIFIC AMERICAN, INC choice was cued by pattern density, however, they departed from the pattern of the normal animals and showed no significant preference [see bottom illus tration at right]. The behavior of dark 2 Teared rats thus resembles that of the day-old chicks, which also lack visual experience. It seems likely, therefore, that of the two cues only motion par allax is an innate cue for depth dis crimination. Responses to differential pattern-density may be learned later. O ne cannot automatically extrapolate these results to other species. But experiments with dark-reared kittens in dicate that in these animals, too, depth perception matures independently of Vi UJ I u � 4 I b: UJ Cl < ::::> I- U « trial and error learning. In the kitten, 8 :however, light is necessary for normal visual maturation. Kittens reared in the dark to the age of 27 days at first crawled {)r fell off the center board equally often {)n the deep and shallow sides. Placed upon the glass over the deep side, they ·did not back in a circle like normal kit tens but showed the same behavior that they had exhibited on the shallow side. Other investigators have observed equiv alent behavior in dark-reared kittens; they bump into obstacles, lack nOlIDal ANIMALS DESCENDING (PER CENT) CONTROL EXPERIMENT measured the effect on rats of reflections on the glass of the apparatus. The percentage of animals leaving the center board decreased with increasing depth i.n much the same way, whether glass was present (black curve) or not ( co lored curve) . eye movement and appear to “stare” .straight ahead. These difficulties pass 100 —r-‘l— after a few days in the light. We accord ingly tested the kittens every day. By the end of a week they were performing in every respect like normal kittens. 80 They showed the same unanimous pref erence for the shallow side. Placed upon the glass over the deep side, they balked and circled backward to a visually se cure surface. Repeated descents to the deep side, and placement upon the glass during their “blind” period, had not taught them that the deep side was “safe.” Instead they avoided it more and more consistently. The initial blindness of dark-reared kittens makes them ideal subjects for studying the mat uration of depth perception. With further study it should be possible to determine which cues they respond to i=’ Z UJ U a< UJ