The Neural Control of Vision
L. The Processing of Depth

As shown in Figure 55, the mechanism of stereopsis arises by virtue of the fact that the two eyes have slightly different views of the world. Stimuli that fall along the horopter (the Vieth-Muller circle), impinge on corresponding retinal points. Those that fall inside or outside of this region, as shown on the right, hit non-corresponding retinal points. This disparity, as long as it is small, can then be utilized by the nervous system to compute the depth difference among various objects, possibly by having non-corresponding points in the two retinae converge on single cells in the visual cortex as depicted in the figure.