Visual localization of the center of mass of compact, asymmetric, two-dimensional shapes

This study aimed at understanding how visual information is used to locate the center of mass. The center of mass is an important physical property of objects that must be taken into account when grasping and/or manipulating them. Participants were instructed to identify the point of equilibrium of compact, bidimensional, massless shapes displayed on a touch screen. The point of equilibrium was defined as the point on the face of the object that would allow one to balance the object in the horizontal position. Seven different triangles and 18 different quadrilaterals in different orientations were used as stimuli. It was found that participants can accurately and consistently estimate the position of the center of mass. The small observed errors were systematically influenced by the shape of the object. The participants tended to locate the center of mass at the center of an inscribed circle instead of the true center of mass. In general, the shape effect was impervious to the orientation of the figure and to the mode of response (left hand, right hand, or mouse).