Binocular Vision and Stereopsis

Animals such as rabbits and horses have two eyes, one on each side of their heads (binocular vision) that provide them a panoramic view and awareness of the world all around them. For instance, rabbits are able to see potential predators approaching from behind because of the lateral position of their eyes. Humans and a number of other species have forward-facing eyes that do not allow for a view behind the head. What evolutionary advantage could make up for the sacrifice in field of view? One enormous advantage is that the two forward-facing eyes receive images of the same portion of the visual world, but the left and right eye images differ because the eyes view from slightly different positions. These image differences on the two retinas (called binocular disparity) are used by the brain to provide a precise estimate of the depth of objects in the world, relative to where the eyes are looking. This transformation of two images into the perception of depth is called stereopsis (Greek for solid sight), and for animals that seek camouflaged prey that are able to remain still, stereopsis can be essential.

Modern humans do not typically use binocular disparity for hunting prey; however, they do rely on this depth cue for daily tasks such as navigating stairs, reaching for the handle of a coffee cup, or threading a needle. Without stereopsis, such tasks become surprisingly difficult; it is only through time and practice that someone who loses stereopsis late in life becomes adept and confident by relying on other, monocular, cues to depth. In conversations with individuals who do not have stereopsis, a common complaint is that, even though long periods have elapsed since they lost stereopsis, they find it difficult to catch a baseball, or they might feel slightly unstable going down stairs. Such losses are experienced by people who once had ste-reopsis, but then are forced to function without it because of sudden illness or injury. In contrast, many individuals who are born without stereopsis are able to rely on other cues, such as motion parallax, to perform daily tasks and even play competitive sports such as cricket. Unfortunately, others suffer permanent visual deficits, such as amblyopia [Page 209](often called lazy eye), resulting from the early disruption of their binocular vision.

Although stereopsis is one of many potential cues to depth used by the brain, it is the only one that, on its own and without head movement, provides such a compelling sense of the volume or space between points in the world. It is not known whether individuals who lack stereopsis experience the space between objects in the same way as people with stereopsis do. In a recent case study, Oliver Sacks recounts the story of a woman named Sue who regained stereopsis while in her 40s. She describes the difference in her world, and in her experience of visual space. In one instance, Sue describes eating a meal, looking at her fork and suddenly, for the first time, being aware of the space between the fork and the food she was eating. Her description of this and other experiences of the space between objects and surfaces provides rare insight into a world devoid of stereoscopic depth.

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