Perceived Size of Rhombuses and Rectangles

Abstract

The purpose of the study was to evaluate the influence of the stimulus shape on the object-size illusion. Rhombuses, rectangles, and hybrid stimuli (elongated hexagons) were presented on the monitor screen as the shapes of blank contours or filled blocks. All stimuli were equal in length and height (180×72 arc min). The rhombus apex angle was 44 deg. The angle of the peak apexes of the hybrid stimuli was changing by steps in the 49-148 deg. range.As the angles were increasing in presentations, the diagonal edges, consequently, were shortening and the horizontal one’s lengthening. In all subjects’ data, the illusion of rhombuses was weaker (2-10%) than that of rectangles (5 to 30%). For some individuals, the rhombuses caused a negative illusion value. The hybrid stimuli produced medium-sized errors that can be arranged in an ascending order according to the apex angle growth: as the shape of the stimulus approached the rectangle, the illusion was increasing, and as it approached the diamond, the illusion strength was decreasing. For the blank and filled stimuli, the illusion magnitude orders were similar to each other, and the curves practically overlapped. To conclude, the angle component in the stimulus contour suppressed the stimulus-size-expansion effect.