Wheel-generated motions have served as a touchstone for discussion of the perception of wholes and parts since the beginning of Gestalt psychology. The reason is that perceived common motions of the whole and the perceived relative motions of the parts are not obviously found in the absolute motion paths of points on a rolling wheel. In general, two types of theories have been proposed as to how common and relative motions are derived from absolute motions: one is that the common motions are extracted from the display first, leaving relative motions as the residual; the other is that relative motions are extracted first leaving common motions as the residual. A minimum principle can be used to defend both positions, but application of the principle seems contingent on the particular class of stimuli chosen. We propose a third view. It seems that there are at least two simultaneous processes—one for common motions and one for relative motions—involved in the perception of these and other stimuli and that a minimum principle is involved in both. However, for stimuli in many domains the minimization of relative motion dominates the perception. In general, we propose that any given stimulus can be organized to minimize the complexity of either its common motions or its relative motions; that which component is minimized depends on which of two processes reaches completion first (that for common or that for relative motions); and that the similarity of any two displays depends on whether common or relative motions are minimized.

In a recent article in this journal, Gogel (1980) argued from two experiments that motion parallax is a result of perceptual underestimation of depth between objects. Gogel also made the more far-reaching assertion that, without some perception of distance, the perception of motion would be impossible. In the present paper, we (1) challenge Gogel's conclusions about motion parallax, (2) criticize his assumption that distance perception is required in perceiving motions, and (3) consider implications for his previous attempts to infer apparent distance from measures of apparent motion (e.g., Gogel, 1977).

A new apparatus for studying the visual perception of rotational motion is presented. The revolving X-Y plotter allows the observer to directly affect changes in the motion parameters of a revolving stimulus as it is being viewed. The advantages afforded by this apparatus over previously used response measures is discussed followed by a description of its design and utilization. The primary value of the apparatus is that it makes possible a precise response measure of the perceived center of any rotating stimulus.

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.

Observers appear to perceive the paths of abstract centers of point-light configurations in making judgments about movement. For configurations on rolling wheels a metric was derived that described the relative vertical motion of this point. It was hypothesized that the smaller the metric the more the stimulus should appear to move in a wheel-like manner with linear translation. In two experiments observers viewed pairs of stimuli and were asked to select either the event that appeared most wheel-like or the one that hopped the most. Viewers consistently selected the stimulus with the smaller metric as being more wheel-like, with a frequency that increased with the difference between metrics. The inverse of this pattern was found for those observers requested to select the stimulus that hopped most. In a second set of two experiments observers drew the translationl paths of these stimuli. Their drawings corresponded to the motion paths of configural centroids. Together, these results strongly suggest that observers perceive the translational component of the motion of the configurations as the path described by their centroids, or geometric centers. We propose that this description of the stimulus event is determined by the logical ordering of information extraction provided by the perceptual system, and discuss this logic and cases where it seems evident.

Undergraduates observed configurations of point-lights undergoing wheel-generated motions and judged how wheel-like the movement of each stimulus appeared on a 7-point scale. Viewer judgments were predicted by a metric defining the variable parameters for the motion path of each configuration’s geometric center—the centroid. The effects on judgments of eye movement and the stimulus characteristics of rotation, translation, and configuration were explored in six experiments. First, a strain operation on the dynamic stimuli did not affect the ability of the metric to predict perceptual judgments. Second, the predictive strength of the metric did not interact with the type of eye movements used in viewing the stimuli, though judged wheel-likeness was greater under pursuit vision than under static fixation. Third, variations in the extent of translation yielded little, if any, effect on observers’ judgments, nor did translation in a circular path. Finally, for stimuli having two lights extremely close together in the configuration, the metric’s predictive value was slightly lessened but only at the limits of visual acuity. Thus, within a wide range of presentation conditions, and for a wide variety of configurations, a metric that defined the variable parameters for the motion path of the centroid was an accurate predictor of observers’ judgments of goodness of perceived rotary motion.