How We Avoid Collisions With Stationary and Moving Obstacles

When moving through cluttered environments we use different forms of the same source of information to avoid stationary and moving objects. A stationary obstacle can be avoided by looking at it, registering the differential parallactic displacements on the retina around it during pursuit fixation, and then acting on that information. Such information also specifies one's general heading. A moving obstacle can be avoided by looking at it, registering the displacements reflecting constancy or change in one's gaze-movement angle, and then acting on that information. Such information, however, does not generally specify one's heading. Passing in front of a moving object entails retrograde motion of objects in the deep background; collisions entail the lamellar pattern of optical flow; and passing behind entails more nearly uniform flow against one's direction of motion. Accuracy in the laboratory compares favorably with that of real-world necessities.