Binocular Cues for Depth

Since your eyes are separated by a few centimeters, the view from one is almost never exactly the same as the view from the other. The left eye sees more of the left side of an object, and the right eye sees more of the right side. These two monocular views are automatically combined into a single subjective view, sometimes called the cyclopean view after the mythical one-eyed Cyclops.

Both the disparities (differences) and the similarities in the two monocular   views produce unique effects in the cyclopean view. If a line or edge is projected onto the same part of the retina in each eye, the contour   will be seen clearly and vividly in the combined cyclopean view, a process called binocular fusion. Exactly which contours will be fused depends on the convergence of the eyes-their rotation inward toward the nose.

Ordinarily you constantly adjust the convergence of your eyes to produce fusion of those aspects of a scene you wish to see clearly. However, when one set of contours is fused by the appropriate convergence, other contours will fall on noncorresponding parts of the retina in each eye. These differences, called binocular disparities, can produce a double image in the cyclopean view or a fluctuating competition,   called binocular rivalry, in which one or the other part of the double image is alternately represented in the cyclopean view. Why aren't we more aware of binocular disparity and rivalry? Primarily because we usually fuse details we are interested in. When you attend to a particular detail, you rotate your eyes to minimize binocular disparity for that detail, thereby producing fusion. You also aim and focus (accomodate) your eyes so that the detail of current interest is clearly projected onto each fovea. If this detail is some distance away, the two eyes may be pointing virtually straight ahead. However, when you attend to something closer, it is necessary to converge the eyes. As you successively attend to details at different distances, you shift convergence, successively producing fusion and disparity for details at each distance. The brain automatically interprets the shifts in convergence, fusion, and disparity as binocular cues indicating relative distance. The whole process, termed binocular   perception or stereopsis, is a major part of our sense of a third dimension (depth) in vision.