Perceived Size of Triangles and Rectangles

Abstract

The aim of the study was the influence of the configuration of the stimulus contour on the illusion of the stimulus size. Horizontally oriented isosceles triangles, rectangles and hybrid stimuli (irregular pentagons) were presented on the monitor screen as empty contour shapes. All stimuli were equal in length and height (180×72 arc min). The triangle apex angle was 23o. The angle of the peak apexes of the hybrid stimuli was changing by steps in the 23o–180o range. As the angles increased in presentations, the diagonal edges of the irregular pentagons became shorter and the horizontal segments grew longer. The subjects adjusted the empty interval of the bipartite stimulus so that it was perceptually equal to the length of the figures, and the mistakes made were considered the values of the illusion of the size of the object. The data obtained showed: i/ the rectangles caused the strongest illusion (9.3%); ii/ the illusion for the triangles was less strong: those the vertices of which were situated at the center of the stimulus caused the 8.5% distortions and the others with vertices at the outer end of the stimulus provided the 3.2% illusion strength; iii/ the illusion of the hybrid stimuli was the weakest (0.7 – 6.3 %). We assume that the global shape, the spatial combination of line segments forming angles and, finally, a closed contour was the main factor of the perceived distortions of the size of the visual stimuli. The positional shifts in the pattern of excitation of an integral contour determined the resulting percept of an object size rather than the sum of shifts in excitations of the separate contour components. The displacements in the excitation pattern might occurred at the level of the grouped associative fields forming the intermediate-level mechanisms of integration of excitations of closed contours. It is well known that the intermediate mechanisms show strong attentional modulation. Att[...].