The Visual Cliff

Gibson, E.J. & Walk, R.D. (April 1960). The "Visual Cliff". Scientific American.

To investigate depth perception in human and animal species, these authors created the "visual cliff" which allowed them to experimentally adjust the optical and tactical stimuli associated with a simulated cliff while protecting the subjects from injury. They discovered that all species can perceive and avoid a sharp drop by the time they take up independent locomotion, be it at Day 1 in chicks, 4 weeks in rats, or 6 months in humans. Most rely on visual cues for depth perception. The rat, however, relies predominantly on tactual cues (being nocturnal) but will fall back on sound vision when needed. Next, the experimenters wanted to find out which visual cues played the decisive role in depth perception. Using dark-reared animals, they concluded motion parallax is an innate cue for depth discrimination, whereas responses to differential pattern-density may be learned later.

(I, Doug G, am the author of this article, The Visual Cliff, and I release its content under the terms of the GNU Free Documentation License, Version 1.2 and later.)