KeywordsDepth Perception Nonhuman Animal Motion Parallax Binocular Disparity Binocular Field
Binocular disparity is a binocular depth cue produced by a difference in retinal projection of the same object onto left eye and right eye retinas as a result of a horizontal separation of the eyes.
Use of Binocular Disparity for Depth Perception in Humans and Nonhuman Animals
When a person looks at a certain point, their eyes fixate on that point, and its projection falls directly onto a fovea of each eye (Fig. 1). Because of the lateral separation of the eyes, the objects in front of the fixation point and behind it will project to a slightly different points on the retina; for example, the retinal distance between points A and F in Fig. 1 is smaller for the left eye than for the right eye (Ponce and Born 2008). This binocular disparity serves as a basis for stereopsis.
Although binocular disparity has been most actively studied in context of depth perception, it also has other benefits. For example, stereoscopic use of binocular disparity improves detection of camouflaged animals against a background that is similar in color and in texture (Wardle et al. 2010). Binocular comparisons also aid in recognition of occluded objects by assisting in discriminating true object borders from the borders created by occlusion (Nakayama et al. 1989).
Although many insects have two or more eyes, their eyes are immobile and have a fixed focus; thus, most insects are unlikely to be able to use binocular disparity for depth perception and must rely on other cues such as motion parallax (Kral 2003). Relatedly, a few insects with a clearly established stereopsis appear to use it in a much more limited fashion than humans (e.g., for estimating distance to prey but not its size; Nityananda et al. 2016).
Binocular disparity in humans is an undoubtedly important cue aiding depth perception; anecdotal reports from the patients who underwent a successful vision recovery emphasize a dramatic improvement in their ability to perceive depth and to detect object boundaries (Ponce and Born 2008). In contrast, the evidence for extensive use of binocular disparity in depth perception in nonhuman animals is limited, with some authors suggesting that monocular depth cues such as motion parallax may play a more important role, even in species with a considerable binocular overlap (Kral 2003; Martin 2009).
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