Size Illusion in Dependence of Stimulus Area

Abstract

We studied the manifestation of the size illusion for four filled geometric shapes: squares, circles, rhombuses and equilateral triangles. In psychophysical experiments, the subjects adjusted the empty interval of the two-part stimulus so that it was equal to the width of the figures the area of which varied within the 0.08 – 10.24 deg2 interval. According to the averaged data for four subjects, the strength of the illusion gradually increased with an increase in the area of squares and triangles, but decreased with an increase in the area of circles and rhombuses. The data obtained can be explained by an interaction of two opposite physiological processes: i/ an increase in the size of an object and ii/ a reduction in distance between two terminal points. Presumably, the expansion of the perceived width occurred for all four figures due to positional shifts in the profiles of the neural excitations caused by the boundaries of the objects. The larger the object was, the greater the positions shifted. But the expansion effect was only apparent for the squares and triangles. In the rhombuses and circles, the lateral segments of the contours played the role of two pairs of inner Müller-Lyer wings, due to which the distance between the endpoints was reduced, therefore, the expansion effect was greatly decreased. The resulting misperception weakened or even became negative when the area of the rhombuses and circles increased. The reduction in distance was greater for rhombuses than for circles, assumingly due to the straight lines of the wings. There was only one pair of wings in the triangles, so the effect of shortening the distance was present but weaker than the effect of expansion. Conclusions: A. The shape of the contour of an object is the main factor determining the strength and sign of the distortions of the perceived size of visual objects. B. The effect of distance reduction may prevail the phenomenon of the size expansion in strength.