Binocular Cues (Physiological Cues)

Some important cues to depth perception in three-dimensional space are provided jointly by both eyes; three of these have been found to be particularly interesting.

Retinal or binocular disparity: this occurs because our two eyes are located at slightly different positions in the head, separated from each other horizontally by a distance of about 6.5 centimetres. Because of this separation, the image of a given object formed on the retina of each eye is slightly different -- this difference between the two images is called retinal disparity. The brain interprets a large retinal disparity as indicating that the object is close by, and a small retinal disparity as indicating that the object is farther away, since the disparity is naturally smaller for distant objects and greater for near objects.

Convergence: when we look at a nearby object, our eyes converge inward in order to bring the object's image onto the fovea of each eye. A group of eye muscles send messages to the brain that indicate the degree to which the eyes are turning inward, and these messages are interpreted by the brain as a cue to depth perception. The degree of convergence decreases progressively as an object moves farther away from the observer. This effect can be experienced directly by holding a finger in front of the nose and slowly bringing it closer: the more the eyes turn inward, or converge, the nearer the object appears to be in space. …