Perceiving the centroid of curvilinearly bounded rolling shapes.

Although its absolute motion is the same, the path of a single moving point is often perceived differently when viewed alone than when viewed as a part of a dynamic configuration of many points. A point’s perceived motion path depends on the configuration in which it is embedded, with different configurations producing different perceived paths. The results of our research suggest that the perceptual system selects the motion path of the configural centroid as the common motion for curvilinearly bounded rolling shapes. To understand better how the perceptual system derives structure from dynamic configurations, we have examined the perception of wheel-generated motions (Proffitt & Cutting, 1979, 1980; Proffitt, Cutting, & Slier, 1979; Cutting & Proffitt, Note 1), a phenomenon with a long history of study from mathematics and philosophy through Gestalt psychology. Like others (Duncker, 1929/1938; Johansson, 1950, 1973; Rubin, 1927; Wallach, 1965), we reduced the stimulus event to configurations of lights on unseen rolling wheels. Figure la shows a single point, P, moving as if attached to a wheel. The curve that describes its path is a cycloid. When a whole wheel is viewed in motion, however, cycloidal motion is not seen. Instead, the perceived motion components are circular rotation and linear translation. These components are also seen if two or more points are placed on an unseen rolling wheel with their configurational centroid at the wheel’s center. Figure lb represents this perception for two points mounted 180 deg apart. Our previous research, employing configurations of two, three, and four lights, provided evidence that perceived components of motion are derived by a logically ordered extraction of information. First, motions of the individual lights relative to