Visual processing of four kinds of relative motion.

Evidence is presented supporting the idea that the human visual system has several specific sensitivities to different kinds of relative motion. These specific sensitivities include: sensitivity to a velocity difference between two different points A and B on one eye's retinal image, the two velocities being directed along the line AB; sensitivity to the velocity difference at A and B between velocity components perpendicular to the line AB (i.e. shearing motion); sensitivity to rotary motion; sensitivity to the ratio between the velocities of the left and right retinal images of an object that is moving in depth. These specific sensitivities can be attributed to relatively hardwired neural filters that are "tuned" to different retinal image correlates of the three-dimensional structure and motion of solid objects in the environment. Such filters may be of use in distinguishing rigid nonrotating objects from nonrigid or rotating objects. They may also be of use in recovering information from the two-dimensional retinal image, including information about object boundaries, the three-dimensional structure of the environment, self-motion and object motion in depth. An alternative way of regarding certain of these specific sensitivities is that they might provide rough physiological equivalents of the values of div V and curl V at every point in the instantaneous velocity field of the retinal image and thus crudely analyze the retinal image flow pattern in terms of mathematical quantities that have the useful property of being relatively invariant against bodily translations of the whole retinal image caused by eye rotation.