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Animal Depth Perception

Cats jump onto tables, horses jump over fences, birds land on window ledges, and chameleons can pick insects out of the air with their long tongues. All of these behaviors, and any others that involve moving around in an environment, require the ability to judge depth and distance. Because depth perception is such an important survival skill, evolution has provided almost every species with a repertoire of ways to evaluate how far away a visual object might be. This entry describes cues for depth, assessing animal depth perception, and animals' use of depth cues.

Cues for Depth

Information about depth and distance is available from several external depth cues in the environment and from internal cues that rely on physiological mechanisms to calculate the position of an object. For the most part, the external cues are referred to as monocular, or pictorial, cues because they can be seen using just one eye, and because they provide the impression of depth that one sees when looking at photographs or paintings. These cues include linear perspective, changes in the density of surface texture, shading, and relative size.

A second class of cues relies on the physiological processing of information created by stimuli that are at different distances. In effect, neurons in the visual cortex compute distance based on the location and movement of images across the retina. Of these physiologically based cues, the most important is retinal disparity, which is the slight difference in the position of images on the retinas of the two eyes that is a consequence of the fact that the eyes are separated on the face. The ability to make use of retinal disparity is known as stereopsis and is the basis for the strong sense of depth that we get when watching a three-dimensional movie wearing the appropriate kinds of glasses.

Assessing Animal Depth Perception

A person can say if he or she sees something in depth and how far away it is. It is much more difficult to measure whether an animal has depth perception, and if so, how good it is. Researchers have used a variety of techniques to assess animal depth perception, each one tailored to a specific species. All share some common principles, however: An animal is taught to make discrimination between two visual stimuli and is rewarded for responding to the correct one. If an animal can make this discrimination, the task is made progressively more difficult until performance falls to chance. This gives a measure of the fineness of the animal's discrimination abilities. For example, several years ago, Donald Mitchell and his colleagues developed the jumping stand technique in which kittens jumped from a raised platform onto a surface below. For measuring depth perception, they used two patterned surfaces, one of which was closer than the other. By reducing the difference in distance, they were able to measure the kittens' depth thresholds. For larger animals, such as the horse, a wide board with two trapdoors can be used, and the horse is required to press its nose against the trapdoor to indicate its choice. In an experiment to measure stereoscopic depth perception in falcons, Robert Fox trained birds to fly from a perch to the visual stimuli that were a short distance away.

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