The Neural Control of Vision
L. The Processing of Depth

Figure 58 provides a demonstration of how differential velocity of movement induces the perception of depth. When stationary, what you see in this figure is just a bunch of random dots. Now, right click on the picture and select play. Repeat to stop. The result is that the display begins to rock back and forth. Almost immediately you can perceive a three dimensional, rigid object consisting of 3 layers. This is accomplished by having the dots in the second and third layers move progressively more rapidly and over a greater distance than in the first layer. The movement exactly mimics what would happen if a real three dimensional object of this sort were rocked back and forth.