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 (convergence and divergence).

Contents

1 Depth perception based on binocular vision 1.1 Parallax 2 Binocular vision in animals 3 Rembrandt as a patient without binocular vision 4 See also 5 External links

Depth perception based on binocular vision

When a person stares at an object, the two eyes converge so that the object appears at the center of the retina in both eyes. Other objects around the main object appear shifted in relation to the main object. In the following example, while the main object (dolphin) remains in the center of the two images in the two eyes, the cube appears shifted to the right when it is captured by the left eye's retina. The same cube appears shifted to the left when it is captured by the right eye's retina.

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 . The brain matches objects identified in both eyes to form a single, combined image. This single, combined image is often known as a Cyclopean image, named after the mythical creatures with a single eye.

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.

Parallax

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 and autostereogram. Stereopsis allow us to triangulate distance more accurately than using monocular vision, resulting in vastly improved depth perception.

The brain uses coordinate shift (also known as Parallax) of matched objects to identify depth of these objects. For instance, the cube is shifted significantly, compared to the main object (dolphin). As a result, it can be inferred that the cube is closer to the eyes than the dolphin.

Binocular vision in animals

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 having 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 them, and then put them 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
  • The New England Journal of Medicine (2004/09/16)
  • Was Rembrandt Stereoblind?