MOTION STRUCTURE IN 5-DOT PATTERNS AS A DETERMINANT OF PERCEPTUAL GROUPING

The effect of basic motion structures on perceptual grouping was studi ed with five-dot motion patterns. Four basic motion structures were id entified in terms of proximal common and relative motion vectors. In a forced-choice situation, the observers had to decide to which of two pairs of dots a fifth critical dot seemed to belong. Thus, one of two possible three-dot units was chosen by the observers. The two possible three-dot units defined different motion structures, and the chosen m otion structure was considered to have stronger grouping power than th e alternative structure had. It was found that parallel common motions (perceived translation in the plane) had the strongest grouping power ; these were followed by circular common motions (perceived rotation i n the plane), concurrent relative motions (perceived translation in de pth), and, finally, parallel relative motions (perceived rotation in d epth). The results also suggested effects of proximity and orientation of axis of rotation. It is further argued that the relative grouping power of the motion structures could not solely be interpreted in term s of changes of directions and distances between the dots. Instead it is suggested that vector analysis is a fundamental perceptual activity and that basic motion structures determine grouping power.