Paradoxical retinal motions during head movements: apparent motion without equivalent apparent displacement.

Abstract

Passengers in moving vehicles often see an interesting phenomenon when they fixate one of several objects that are at different distances. Looking sideways out the window, they may notice that all objects closer than the fixated object appear to move in a direction opposite to their movement, whereas all objects farther than the fixated object appear to move in the direction in which they are traveling. You can see similar motions by holding your index fingers at different distances while swinging your head back and forth. When you look at the far finger, the near one appears to move against the direction of head movements; when you look at the near finger, the far one appears to move with the direction of head movement. Theterm parallax is often used in referring to these and other motions that are concomitant with lateral head movements (e.g., Coren, Porac, & Ward, 1979; Gibson, Gibson, Smith, & Flock, 1959; Shebilske & Proffitt, 1981). Gogel, however, prefers the term apparent concomitant motion. This paper will adopt Gogel's term to avoid clouding issues in an ongoing debate that contends issues much deeper than terminology. On one side of the debate, Gogel(1980, 1981, 1982) argues for an apparent-distance/pivot-distance hypothesis, according to which the motions occur because people underestimate the distance between objects. On the other side, Shebilske and Proffitt (1981) argue for a paradoxical-motion hypothesis, according to which motions occur because people experience a paradoxical sensation of retinal motions. This paper will analyze these two hypotheses and their implications for visual-motor coordination, motion perception, and distance perception. One issue in the debate involves a distinction between motion per se and displacement or change in target location. The distinction is important because people sometimes experience paradoxical motion, which is a perception resulting