Perceived Length of Different Shapes

Abstract

Background and aim: Perception of spatial dimensions of visual objects depends on their shape and surface properties1. Rectangular objects, or simply a contour, appear longer than an empty space of the same length that was simultaneously shown next to the stimulus. According to our latest psychophysical data, other geometric shapes (triangles, ovals, or rhombuses) also cause misperceptions of size, but obviously of different strengths and even signs2. Materials and methods: In the present experiments with the length adjustment procedures, subjects adjusted the empty space of the stimulus to equal the length of the test object. The judgment errors indicated the strength and sign of the object size illusion. Six geometric shapes were tested: a simple line, rhombus, oval, rectangle, and two equilateral triangles with the vertex of one directing toward the stimulus center and with that of the other pointing toward the one of the stimulus ends. Filled and empty stimuli were used. Results: The strength of the illusion and the order of the strengths were different among observers. However, some general regularities have been determined. The vertex of both the empty and filled triangle pointing toward the stimulus center was the most powerful shape to induce the illusion of extent. The triangles of the lateral orientation were twothree times less effective than those directed to the center. The rhombus shape was the weakest stimulus type. The rectangles and ovals were about of the middle strength. The lateral orientation triangles and rhombuses might produce the illusion of the negative sign. The filled shapes provided a somewhat stronger illusion than the empty contours. Conclusions: The data obtained support the assumption that the contours of the visual objects are the main primary determinant of the illusion of the object size.