The visual cliff is an apparatus created by psychologists Eleanor J. Gibson and Richard D. Walk at Cornell University to investigate depth perception in human and other animal species. It consists of a sturdy surface that is flat but has the appearance of a several-foot drop part-way across. The visual cliff apparatus allowed them to conduct an experiment in which the optical and tactile stimuli associated with a simulated cliff were adjusted while protecting the subjects from injury. Using a visual cliff apparatus, Gibson and Walk examined possible perceptual differences at crawling age between human infants born preterm and human infants born at term without documented visual or motor impairments.
Design The visual cliff consisted of a sheet of Plexiglas that covers a cloth with a high-contrast checkerboard pattern. On one side the cloth is placed immediately beneath the Plexiglas, and on the other it is dropped about four feet (1.2 m) below.
Original study Gibson and Walk (1960) hypothesized that depth perception is inherent as opposed to a learned process. To test this, they placed 36 infants, six to fourteen months of age, on the shallow side of the visual cliff apparatus. Once the infant was placed on the opaque end of the platform, the caregiver (typically a parent) stood on the other side of the transparent plexiglas, calling out for them to come or holding an enticing stimulus such as a toy. This gave the infant the motivation to crawl across towards them. It was assumed if the child was reluctant to crawl to their caregiver, he or she was able to perceive depth, believing that the transparent space was an actual cliff. The researchers found that 27 of the infants crawled over to their mother on the "shallow" side without any problems. A few of the infants crawled but were extremely hesitant. Some infants refused to crawl because they were confused about the perceived drop between them and their mothers. The infants knew the glass was solid by patting it, but still did not cross. In this experiment, all of the babies relied on their vision in order to navigate across the apparatus. This shows that when healthy infants are able to crawl, they can perceive depth. However, results do not indicate that avoidance of cliffs and fear of heights is innate.
Infant studies During early development, infants begin to crawl, sit, and walk. These actions impact how the infants view depth perception. Thus, infant studies are an important part of the visual cliff. When an infant starts to engage in crawling, to sit, or walking, they use perception and action. During this time, infants begin to develop a fear of height. The everyday exploration of infants gives them clues about things or objects to avoid when exploring. Other research that has used the visual cliff focuses on preterm infants, prelocomotor Infants, and maternal signaling.
Preterm infants Sixteen infants born at term and sixteen born preterm were encouraged to crawl to their caregivers on a modified visual cliff. Successful trials, crossing time, duration of visual attention, duration of tactile exploration, motor strategies, and avoidance behaviors were analyzed. A significant surface effect was found, with longer crossing times and longer durations of visual attention and tactile exploration in the condition with the visual appearance of a deep cliff. Although the two groups of infants did not differ on any of the timed measurements, infants born at term demonstrated a larger number of motor strategies and avoidance behaviors by simple tally. This study indicates that infants born at term and those born preterm can perceive a visual cliff and change their responses accordingly.
Prelocomotor infants Another study measured the cardiac responses of human infants younger than crawling age on the visual cliff. This study found that the infants exhibited distress less frequently when they were placed on the shallow side of the apparatus in contrast to when they were placed on the deep side. This means that prelocomotor infants can discriminate between the two sides of the cliff.
Maternal signaling James F Sorce et al. tested to see how maternal emotional signaling affected the behaviors of one-year-olds on the visual cliff. To do this they placed the infants on the shallow side of the visual cliff apparatus and had their mothers on the other side of the visual cliff eliciting different emotional facial expressions. When the mothers posed joy or interest most of the babies crossed the deep side but if the mothers posed fear or anger, most of the babies did not cross the apparatus. In contrast, when the visual cliff effect was absent, most of the babies crossed regardless of the mother's facial expressions. This suggests that babies look to their mother's emotional expressions for advice most often when they are uncertain about the situation. Joseph J. Campos research focuses on facial expressions between the caregiver and infant. Specifically his research shows that the infants will not crawl if the caregiver expresses a signal of distress. If the caregiver gives the infant a positive facial expression the child is more likely to crawl across the visual cliff.
Non-human experiments Before Gibson and Walk conducted their study with human infants, multiple experiments were conducted using rats, one-day-old chicks, newborn kids, kittens, pigs, adult chickens, dogs, lambs, and monkeys. Overall, most species would avoid the deep side of the visual cliff, some right after being born. The first visual cliff experiment was conducted with rats who were raised in the dark and in the light. The results were that both groups of rats would walk all over the shallow and deep parts of the cliff without an issue, which surprised Gibson, Walk, and Thomas Tighe (a research assistant). A later experiment with kittens raised in the dark and then placed on the visual cliff showed that depth perception was not innate in all species as the kittens would walk on either side of the visual cliff. After six days of being in the light, the kittens would avoid the deep side of the visual cliff (Rodkey, 2015). Later researchers conducted experiments using other species.
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