The Neural Control of Vision
L. The Processing of Depth

Subsequently it was discovered that a clear impression of depth can also be obtained when two random-dot patterns are presented, one to each eye, in on of which a group of dots is shifted to the left or right. An example of such a display appears in Figure 53. When these two images are viewed through a stereoscope allowing them to be fused, a viewer with intact stereoscopic vision will see a central square region protruding from the rest of the image (see I.P. Howard & B.J. Rogers, Seeing in Depth: Vol 1&2 (2002) Thornhill, Ontario: I. Porteous). Such random dot stereograms are excellent tools for determining whether a given individual possesses stereoscopic vision. This is an important task as people without it are quite common and often are unaware of it. Most individuals who lack stereoscopic vision are found among those who are strabismic from birth. If discovered early in life, corrective surgery in some cases can result in the re-enstatement of stereoscopic vision.