How the Visual Cliff Tested Babies' Depth Perception

A visual cliff involves an apparent, but not actual drop from one surface to another, originally created to test babies' depth perception. It's created by connecting a transparent glass surface to an opaque patterned surface. The floor below has the same pattern as the opaque surface. This apparatus creates the visual illusion of a cliff while protecting the subject from injury.

History of the Visual Cliff

In order to investigate depth perception, psychologists E.J. Gibson and R.D. Walk developed the visual cliff test to use with human infants and animals. Earlier research had revealed that infants will respond to various depth cues even before they are able to crawl.

Depth cues allow people to detect depth in a visual scene. These can include both monocular cues such as relative size and overlap, or binocular cues such as retinal disparity. Gibson and Walk were interested in whether or not an infant's ability to perceive depth is a learned behavior or if it was, as they suspected, innate.

Gibson and Walk described their visual cliff apparatus as a large sheet of heavy Plexiglass supported a foot or more off the floor. Early versions of the experiments involved animal subjects such as turtles, goats, rats, lambs, kittens, dogs, pigs, and monkeys.

On one side of the glass, a high-contrast patterned fabric is pressed up against the underside to make the glass appear solid. The same material is laid on the floor below the glass, creating the visual illusion of a cliff. This allowed researchers to test infant perception while still ensuring the safety of their young subjects.

Visual Cliff Infant Test

In the test, a child is placed on one end of the platform and the caregiver stands on the other side of the clear surface. The assumption was that if a child had developed depth perception, they would be able to perceive the visual cliff and would be reluctant or refuse to crawl to the caregiver. It was also assumed that infants who still lacked depth perception would crawl happily to their caregivers without even noticing the apparent drop.

Gibson and walk concluded that the ability to perceive depth emerges sometime around the age that an infant begins to crawl. The fear of heights, they suggested, is something learned later in infancy as gain experience with bumps, scrapes, and falls.

Understanding the Visual Cliff

Initially, psychologists believed that the perception of the visual cliff was a matter of physical and visual maturity. Babies could see the difference by the age of eight months, while younger infants with less developed depth perception could not see the cliff.

Because six-month-old children could be enticed to wiggle across the visual edge, while 10-month-old children refused to cross the threshold, in 2013 it was assumed that the younger children had not yet developed depth perception while the older children had.

Later research published in 2014, however, has demonstrated that children as young as three months are able to perceive the visual cliff. When placed over the apparent "edge," their heart rates quicken, eyes widen, and breathing rates increase. So if these infants can perceive the visual cliff, why would they be willing to crawl off what appears to be a straight drop down?

The assumption has long been that avoidance of the visual cliff was related to a fear of heights, but recent research suggests that infants avoid the drop-off because they sense that they lack the physical skills to make the descent possible.