Expansion of perceived size of visual stimuli: Objects look wider than equivalent empty spaces.

The study builds upon previous research on the perceived size of visual objects of various shapes compared to an empty spatial interval. In psychophysical experiments using the size-matching procedure, the effect of overestimating the relative size of an object (relative to an equivalent empty space) was consistently observed when testing visual objects, such as rectangles, circles, ellipses, rhombuses, and triangles, in both filled and empty formats. The strength of the illusion did not depend on whether the shapes were filled or not, but rather it varied with the shape itself. Objects with open contours, such as angles of different orientations and narrow stimuli like straight, tangled, defocused, and divided lines, all produced the expansion effect. The overestimation manifested during testing stimuli of various contour types, including spatial contrast of luminance, colour, and texture, as well as those determined by perceptual grouping and illusory outlines of Kanizsa and Oppel-Kundt versions. Finally, the expansion effect was found to be more pronounced with increasing length and height of the stimuli. The data supported the assumption that the object contour is the primary inducer of perceived size expansion and that the overestimation effect is a regular phenomenon rather than an incidental event.