Motion is a sine qua non of life. Because of this property, the visual detection of motion is of great significance, particularly for animals. Potential predators, prey, and mates all move, and, insofar as visual systems can detect this at a distance, they will be of great benefit to their owners. Thus, motion automatically captures our attention and generates orienting responses. Moreover, the motion of things is an excellent predictor of where they will be in the future, thus allowing animals to anticipate trajectories and program appropriate movements to either acquire or avoid the moving object. However, because there are many spurious sources of motion—such as those generated by eye, head, or body movements of the observer—distinguishing “real,” object-based motion from self-generated motion is a ...