Binocular vision (also referred to as stereoscopic vision) is a type of visual system common in many kinds of animals where both the eyes produce only a single image in the brain. It may be contrasted with monocular vision, where the information from each eye is processed separately. Most humans have binocular vision, unless they are affected by blindness, differential focus, strabismus or a related condition.

In binocular vision, the eyes are forward-facing and usually move together. These eye movements can be to change the direction of gaze, in which case both eyes move in the same direction, called a version, or to change the distance of gaze, in which case both eyes move in opposite directions, called a vergence.

Because each eye is in a different horizontal position, each has a slightly different perspective on a scene yielding different retinal images. Normally we do not see these two images, but see a single view of the scene, a phenomenon known as singleness of vision. Yet if the images are very different (such as by going cross-eyed, or by presenting different images in a stereoscope) we may see one image at a time, a phenomenon known as binocular rivalry.

When viewing normally, the differences between the positions of the two eyes mean that the same part of an object is projected onto different horizontal positions in the two retinal images. This is called retinal disparity or horzontal disparity and is the basis of stereopsis. Stereopsis allow us to triangulate distance more accurately than using monocular vision, resulting in vastly improved depth perception. Binocular vision contributes to depth perception at close distances, within 18-20 feet; beyond that, the brain relies on less precise cues, such as shadows and (especially) the relative motion of objects at different distances to generate depth information.

Animals in which binocular vision has been disabled (e.g. through accident to one of the eyes) may compensate for the loss through motion of the head; these shifts in perspective provide a rudimentary sort of binocular vision by exploiting motion parallax.

Binocular vision is a feature common among many hunting animals, but also among primates which rely upon it when navigating complex three-dimensional environments.

Binocular vision comes at the expense of a wider field of view, meaning that an animal must rely on other senses to see what is behind it or on the periphery. For many prey species, like cows or horses, the wider field of view given by side-facing eyes and monocular vision is a better adaptation, because it reduces the chance that a predator could sneak up on them.

Rembrandt as a patient without binocular vision

People whose eyes failed to align correctly could be unable to form binocular vision. However, to an artist, this disability could have been a gift.

In an article published on September 16, 2004 in The New England Journal of Medicine, Margaret S. Livingstone, professor of neurobiology of Harvard Medical School, suggests that Rembrandt (July 15, 1606-October 4, 1669), generally considered one of the greatest painters in European art history, suffered from this disability. She made this conclusion after studied 36 of Rembrandt's self-portraits.

Because Rembrandt could not form a normal binocular vision, his brain automatically switched to one eye for many visual tasks. This could have helped him to flatten images as he saw, and then put it onto the two-dimensional canvas. In the author's words:

Art teachers often instruct students to close one eye in order to flatten what they see. Therefore, stereoblindness might not be a handicap -- and might even be an asset -- for some artists.

See also

• Stereoscopy
• Vision
• Binocular rivalry
• Rembrandt

External links

• Deconstructing the Gaze of Rembrandt (http://www.nytimes.com/2004/09/16/arts/design/16remb.html)
  • The New England Journal of Medicine (2004/09/16) (http://content.nejm.org/content/vol351/issue12/index.shtml)
  • Was Rembrandt Stereoblind? (http://content.nejm.org/cgi/content/short/351/12/1264)

pl:Widzenie stereoskopowe