Stereoscopic Vision

Humans and many other animals have stereoscopic vision, in which they see an object with two eyes at the same time, with both eyes set in the same plane. This is not true of all animals: in most fishes each eye, located on opposite sides of the head, generates an independent image.

This type of vision is important in depth perception, and animals that have it usually need the ability to judge distances nicely. The images formed by the two eyes and transmitted to the brain's visual cortex are not quite identical: the distance between the two eyes means that they see an object from slightly different angles. The principle is exactly the same as that used in the "rangefinder" mechanism of a camera, in which a rotating mirror is adjusted to make its image coincide with that viewed directly through the viewfinder, and the distance is read off from a scale calibrated to the angle of rotation. The brain does the same thing; it is capable of comparing the two images and judging the amount of parallax the "baseline" between the eyes has produced, and rendering a single integrated image in three-dimensional view.

Individuals who have lost the vision in one eye have to develop other mechanisms to compensate for the loss of parallax as a means to judge distances, and often have poor depth perception, regardless of whatever adjustments they make. The human ability in stereoscopic vision is closely related to their fine manipulation skills. In arboreal animals that move from branch to branch, for obvious reasons, there is also stereotactic visual ability.

The squirrel has eyes on opposite sides of its head, but the two produce visual fields that overlap. This is sufficient to permit a squirrel to leap from branch to branch. Many small birds that perch on tree branches and flit through shrubbery have a similar arrangement. Owls are the exceptions among birds; they are the only group of avians whose eyes are located in the same plane, and they have binocular vision in the same sense that flat-faced animals like primates do. Many birds have eyes on the sides of their heads, and are unable to view a single object with both eyes. The chicken and the pigeon are two good examples of this. Such birds must judge distance by moving their heads and viewing an object with each eye independently, deducing from the displacement how far away the object must be. (A human can get an idea of this effect by viewing an object at a fixed distance with each eye alternately.)

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