The Neural Control of Vision
L. The Processing of Depth

A monocular cue of great power for depth perception is motion parallax. This mechanism arose to utilize a basic rule about motion: The rate at which rigidly linked objects move in the visual field (especially when one is in motion in the world) depends on their distance relative to where one fixates. Objects closer than the fixation site move faster and in the opposite direction than do objects beyond fixation. This is depicted in Figure 57. The rigid object is a pole with fixation at white dot in the center of the pole. As the pole is moved, the observer tracks the dot. The movement of the square and triangle at the two ends of the pole are depicted on the retinal surface. To utilize motion parallax neurons are needed in the brain that can tell you not only about direction of motion but also about the relative velocity of the moving stimuli. Such neurons have been reported to exist in V1 and MT. Other areas may soon follow.