From local data to global conclusions

Figure 6.13: Due to local nature of motion detectors, the aperture problem results. The motion of the larger body is ambiguous when perceived through a small hole because a wide range of possible body motions could produce the same effect inside of the hole. An incorrect motion inference usually results.

Motion detectors are local in the sense that a tiny portion of the visual field causes each to activate. In most cases, data from detectors across large patches of the visual field are integrated to indicate coherent motions of rigid bodies. (An exception would be staring at pure analog TV static.) All pieces of a rigid body move through space according to the equations from Section 3.2. This coordinated motion is anticipated by our visual system to match common expectations. If too much of the moving body is blocked, then the aperture problem results, which is shown in Figure 6.13. A clean mathematical way to describe the global motions across the retina is by a vector field, which assigns a velocity vector at every position. The global result is called the optical flow, which provides powerful cues for both object motion and self motion. The latter case results in vection, which is a leading cause of VR sickness; see Sections 8.4 and 10.2 for details.