The visual cliff apparatus was created by psychologists Eleanor J. Gibson and R.D. Walk at Cornell University to investigate depth perception in human and animal species. This apparatus allowed them to experimentally adjust the optical and tactical stimuli associated with a simulated cliff while protecting the subjects from injury. The visual cliff consists 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 4 feet below. Since the Plexiglas supports the weight of the infant this is a visual cliff rather than a drop off. 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.