Local versus global perception of ambiguous motion displays.

Four pairs of spots rotating around their common centers, each pair a "doublet", were arranged in a square formation. The doublets initially appear to rotate "locally", but then coalesce into two large squares that slide over each other along circular paths ("global" motion). This perceptual transition from local to global motion occurred within and across a series of repeat trials (Experiment 1). Doublet members set close or far apart promoted local or global motion respectively. Increasing the number of spots within a group from two to three or four made the motion look more local. Spots of the same luminance polarity against a gray background tended to move together (Experiment 2); and when replaced by short black lines of different orientations, lines of the same orientation tended to move together, favoring rigid rather than shearing motions (Experiment 3). The perceived trajectories of spots tended to follow static circles or dynamic contours painted on the screen (Experiment 4). These findings suggest the visual system parsimoniously attempts to group the maximum number of moving spots into the minimum number of groups.