Encyclopedia of Evolutionary Psychological Science

Living Edition
| Editors: Todd K. Shackelford, Viviana A. Weekes-Shackelford

Binocular Disparity

  • Olga LazarevaEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-319-16999-6_2760-1


Depth Perception Nonhuman Animal Motion Parallax Binocular Disparity Binocular Field 
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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.


Stereopsis, or the perception of the “true” location of the objects in depth obtained on the basis of binocular information, requires the two eyes to be located at different lateral positions so that they receive slightly different projections onto their retinas when focused on the same point in space (Fig. 1). In addition, stereopsis requires coordinated eye movements; disorders that interfere with this coordination such as amblyopia and strabismus lead patients to report perceiving two images of a single object (diplopia or double vision; Otero-Millan et al. 2014). Although the importance of binocular disparity in achieving depth perception in primate vision is well established, its role in vision of other animals is less clear, particularly for animals with laterally located eyes.
Fig. 1

Geometry of binocular disparity and stereopsis. As both eyes simultaneously fixate on a point F, it falls on their foveae. The point A lies closer to the observer (i.e., before the point of fixation) than the point B; therefore, the projections of these points fall on different locations in the left and the right eyes. Consequently, the distance from the fixation to the projection of each point (denoted as a–f or b–f) will be different for the left and the right eyes; this difference is termed binocular disparity and can be used for recovering depth information

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).

All vertebrate animals have two eyes, and some degree of binocular overlap is present even in animals with laterally located eyes (Fig. 2). However, the function of this binocular overlap and its involvement in depth perception remains a subject of a lively debate, particularly in birds. Although some authors assert that avian binocular field produces stereoscopic depth perception, just as it does in primates (e.g., Nieder 2003), others argue that binocular disparity in avian vision is used primarily for in control of a beak placement and during visual inspection of items held in the beak (Martin 2009). Instead, depth perception in birds is proposed to rely on motion parallax produced by repetitive head movements (Kral 2003). The relatively large binocular field of owls (Fig. 2) is argued to be a byproduct of enlarged eyes and elaborate outer ears that prevent lateral placement of the eyes, an argument supported by a much smaller binocular field found in other birds of prey (Martin 2009). Unfortunately, global stereopsis (a process involving processing and comparison over a large portion of retina) has been convincingly demonstrated in only one bird species, barn owl (van der Willigen 2011), leaving the argument unresolved. In contrast, motion parallax has been shown to produce reliable depth perception in both humans and nonhuman animals (Kral 2003).
Fig. 2

Diagrams of visual fields of humans and three bird species with the eyes at rest (with the bill or the nose facing forward). Pigeon and American kestrel have laterally located eyes, whereas tawny owl has frontally located eyes (The avian visual fields are adapted from Martin (1994))

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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© Springer International Publishing AG 2017

Authors and Affiliations

  1. 1.Drake UniversityDes MoinesUSA

Section editors and affiliations

  • Russell Jackson
    • 1
  1. 1.University of IdahoMoscowUSA