Gibson, E. J., & Walk, R. D. (1960). The “visual cliff.” Scientific American, 202(4), 64-71.
The visual cliff experiment (Gibson & Walk, 1960) tested whether depth perception is innate. Researchers placed infants and newborn animals on a glass-topped table that created the illusion of a sudden drop, then recorded whether they would cross it.
Most infants and animals refused, suggesting that depth perception develops very early in life rather than through learning.
Aim
Eleanor J. Gibson and Richard D. Walk (1960) investigated whether newborn animals and human infants could detect depth.
Gibson and Walk tested whether infants and young animals would crawl over an apparent drop. If they did, this would suggest that depth perception is not present from birth.
If they refused, this would support a nativist view: that perceptual abilities are innate. Six months old was the earliest age at which human infants could be tested this way, since the procedure depended on independent crawling.
Key Takeaways
- The Apparatus: A safe glass-topped table creates the illusion of a sudden drop, letting researchers test depth perception without any real risk of falling.
- Human Findings: About 92% of 6- to 14-month-old infants refused to crawl onto the “deep” side even when their mother called them from across it.
- Animal Evidence: Newly hatched chicks, goat kids and lambs avoided the deep side completely from birth, supporting an innate explanation.
- The Crawling Confound: Because infants could only be tested once they could crawl, around six months, the study alone could not rule out earlier learning.
- Resolving the Confound: Later research measuring heart rate in two-month-olds (Campos, Langer & Krowitz, 1970) found they could already perceive the depth difference, long before they could crawl.
- Perception vs. Fear: Depth perception itself appears very early. Wariness of a drop builds up later, tracking an infant’s own movement experience (Dahl et al., 2013).
- A Fuller Picture: A historical review (Rodkey, 2015) found Gibson and Walk tested far more species than the popular chicks-goats-lambs story suggests, with mixed results.
Procedure
Gibson and Walk built a heavy glass-topped table, about four feet high, with a sturdy board running across its centre. A crawling infant or animal could be placed on this central board with no possibility of actually falling.
On one side, the “shallow side”, a chequered pattern sat directly beneath the glass, so the surface looked solid.
On the other, the “deep side”, the same pattern sat several feet below on the floor. This created the visual impression of a sudden drop, even though the glass itself stayed unbroken across both halves.
Experiment 1
Sample: 36 infants ranging in age from six months to 14 months. Their mother also participated in the experiment.
The independent variable (IV) was whether the infant was called by its mother from the cliff side or the shallow side (of the visual cliff apparatus).
The dependent variable (DV) was whether or not the child would crawl to its mother.
This was a repeated measures design because the infant was called from both the cliff side and the shallow side of the apparatus.
Experiment 2
Using the same apparatus, Gibson and Walk tested chicks, lambs, and kids (young goats), all less than 24 hours old.
The studies using other species are quasi (laboratory) experiments. The naturally occurring independent variable (IV) was the animal species, e.g., rat/chick/lamb/kitten.
The dependent variable (DV) was whether the animal preferred the shallow side or the deep side of the visual cliff apparatus.
When animals were placed directly onto the deep side by hand, they typically froze into a defensive position rather than moving at all. Repeated trials of this kind showed that the animals did not learn, over time, that the glass was safe to stand on.
Other species were also tested, including rats (which were additionally tested with a raised bridge) and kittens, which were several weeks old before they could be tested. Some kittens were tested after being reared in the dark.
The rats were also tested with apparatus providing fewer visual cues by replacing the chequered pattern with a uniform grey surface to see whether the pattern was essential to perceiving depth.
Findings
- Human infants: Even when their mother called them, about 92% (33 of the 36) infants refused to cross onto the deep side. The few who did so typically backed onto it by accident rather than crawling forward deliberately.
- Chicks, lambs and kids: None crossed to the deep side. When placed there directly by hand, they froze into a defensive position, even after repeated trials.
- Rats: Rats rely on their whiskers rather than vision to guide movement, so they crossed onto the deep side by default. They only avoided it when the bridge was raised too high for their whiskers to reach.
- Kittens: Kittens tested at about four weeks showed a clear preference for the shallow side. Kittens reared in the dark until 27 days showed no preference at first, but matched light-reared kittens after about a week of light.
Conclusion
Because the infants were able to detect the danger on the “cliff” side, Gibson and Walk concluded that depth perception might be innate. It was, at the very least, present as soon as the infants could crawl.
However, as human infants take several months to crawl, it is possible that they had learned their ability to perceive depth during this time. The second experiment aimed to explore this possibility using animals.
Animals are able to judge depth as soon as they become mobile, whether that is immediately after birth or hatching, or somewhat later. This ability does depend on visual experience, meaning time spent in the light. Even so, recovery from visual deprivation is quick compared with how long it lasted.
Together, the findings suggest that depth perception is an innate process.
Any species must develop depth discrimination by the time its members become independently mobile. For the chick and the goat, this happens within a day. For the rat and the cat, it takes three to four weeks. For the human infant, it takes six to ten months.
Later Research: Resolving the Learning Confound
Gibson and Walk’s human infants were already crawling, so their study could not rule out learning. Two later lines of research closed this gap by testing infants and behaviours that could not have been learned.
Testing Infants Too Young to Crawl (Campos, Langer & Krowitz, 1970)
Aim: Because Gibson and Walk’s method needed infants old enough to crawl, it could not show whether depth perception exists before six months. Campos, Langer and Krowitz tested younger infants using a response that needs no voluntary movement at all: heart rate.
Method: Two-month-old and nine-month-old infants were placed directly onto the shallow or deep side of the apparatus while wearing electrodes that recorded their heart rate throughout.
Results: The nine-month-olds’ heart rate rose on the deep side, a pattern linked to fear. The two-month-olds’ heart rate fell instead, the pattern usually seen with interest rather than distress.
Conclusion: A heart-rate change in either direction shows the infant’s body was registering the difference in depth. Even two-month-olds, who cannot yet crawl or have learned to fear a drop, could tell the two sides apart.
Self-Produced Movement and Learning to Fear Heights (Dahl et al., 2013)
Aim: If fear of drops is learned, what do infants actually learn from? Dahl and colleagues focused on self-produced locomotion, moving one’s own body through space.
Method: Infants too young to crawl practiced steering powered go-carts by joystick for several weeks; a comparison group had no such practice. Researchers then lowered both groups toward a real 1.3-metre drop-off while recording heart rate, and tested them separately in a “moving room” that simulated being pushed forward.
Results: Go-cart-experienced infants showed a clear heart-rate rise (about five beats per minute) toward the drop-off and recoiled from the moving room; infants without practice showed neither response.
Conclusion: Learning to fear heights depends on practicing control of one’s own movement, whether by crawling, walking or driving a go-cart. It does not depend simply on getting older or falling a certain number of times.
Critical Evaluation
Methodological Strengths
The procedure was a rigorously controlled laboratory test, offering a reliable and safe measure of depth perception. Gibson and Walk were able to eliminate or control the influence of other senses, such as touch from the rats’ whiskers, which helped ensure a valid test of visual perception.
Sample Size Limitations
The consistency of the results across a range of species, including humans, adds credibility to the findings. However, the human sample was small, and the six-to-14-month age range was wide enough that some infants had likely been crawling for months before they were tested.
Ethical Considerations
Although the mothers were present and gave informed consent, a potential ethical issue remained. Simply looking at the drop, or being encouraged by their mothers to cross it, may have distressed the babies, since they had no way of knowing the glass would keep them safe.
Contemporary Research
The Forgotten Menagerie (Rodkey, 2015)
Historical work re-examining Gibson and Walk’s original records complicates the tidy chicks-goats-lambs textbook story. Rodkey (2015) traced the original experimental series and found the pair tested a far wider range of species, including rats, aquatic turtles, kittens, pigs, dogs and monkeys.
The results varied by species: aquatic turtles were the weakest avoiders, with about three-quarters choosing the shallow side, since life in water reduces the value of depth discrimination.
Locomotor Experience and Social Information (Karasik et al., 2016)
Aim: Karasik, Tamis-LeMonda and Adolph (2016) asked what determines how infants combine perceptual judgement with a caregiver’s social signal when deciding whether to descend a risky edge. Specifically, they tested whether locomotor experience, rather than age alone, is the deciding factor.
Method: Thirty infants (12-month crawlers, novice walkers, 18-month experienced walkers) were tested on an adjustable drop-off apparatus at their measured “borderline” height, with their mother encouraging or discouraging descent.
Results: Experienced crawlers and walkers used their mother’s message selectively, ignoring it at clearly safe or clearly risky heights but being swayed by it at their own borderline height. Novice walkers often attempted risky drop-offs regardless of her message and lost their balance more often.
Conclusion: Locomotor experience, not chronological age, determines how well an infant blends perceptual and social information about risk. Novice walkers are exposed to real risk precisely because they have not yet learned when to trust their own judgement and when to check in socially.
Perceptual Adaptation in Adults
The investigation of the nature-nurture issue in perception did not end with Gibson and Walk’s research. Their study only explored the plasticity of infant perception, leaving open the question of whether adult perception can also adapt.
This has, however, been investigated in several different ways. Stratton (1897) and Kohler (1962) used complex optical apparatus, such as inverting prisms, to change how their view of the world appeared.
In these situations, the world at first seems upside down or muddled, but the brain adapts over time and normal perception returns.
When the apparatus is removed, it again takes time for perception to revert. Other, less dramatic changes can also be induced by shifting the field of view slightly to one side. Depth perception is then tested by, for example, how accurately a person can point to a target.
In such situations, people adapt readily, usually within about an hour, but only if they can actively interact with their environment. Participants who are instead pushed around in wheelchairs fail to learn to cope with the visual distortion (Held & Bossom, 1961).
Findings such as these suggest that, at least in some respects, depth perception is learned.
References
Campos, J. J., Langer, A., & Krowitz, A. (1970). Cardiac responses on the visual cliff in prelocomotor human infants. Science, 170 (3954), 196-197.
Dahl, A., Campos, J. J., Anderson, D. I., Uchiyama, I., Witherington, D. C., Ueno, M., Poutrain-Lejeune, L., & Barbu-Roth, M. (2013). The epigenesis of wariness of heights. Psychological Science, 24 (7), 1361-1367.
Gibson, E. J., & Walk, R. D. (1960). The “visual cliff.” Scientific American, 202 (4), 64-71.
Held, R., & Bossom, J. (1961). Neonatal deprivation and adult rearrangement: Complementary techniques for analyzing plastic sensory-motor coordinations. Journal of Comparative and Physiological Psychology, 54 (1), 33-37.
Karasik, L. B., Tamis-LeMonda, C. S., & Adolph, K. E. (2016). Decisions at the brink: Locomotor experience affects infants’ use of social information on an adjustable drop-off. Frontiers in Psychology, 7, Article 797.
Kohler, I. (1962). Experiments with goggles. Scientific American, 206 (5), 62-72.
Rodkey, E. N. (2015). The visual cliff’s forgotten menagerie: Rats, goats, babies, and myth-making in the history of psychology. Journal of the History of the Behavioral Sciences, 51 (2), 113-140.
Stratton, G. M. (1897). Vision without inversion of the retinal image. Psychological Review, 4 (4), 341-360.
