Perceived size of objects with motion-defined contours

Abstract

An increase in the relative size of visual objects (the illusion of expansion) was constantly observed in psychophysical experiments with stimuli that differed in shape, size, and orientation and were outlined by a spatial contrast in luminance, color, or texture, also defined by lines, perceptual grouping, and subjective (illusory). The strength of the illusion monotonously increased from a minimum to about 20 arc min with an increase in the length of the stimuli from 25 to 225 arc min for various contours tested, except the Kanizsa type. The latter caused twice weaker effects: the maximum value approached 10 arc min. Thus, subjective contours provided a weaker spatial signal than real ones. The contour may not be a uniform condition for the illusion. To test the prediction, the following type of contour was taken for experiments - defined by motion. Borders of the rectangular-shaped stimulus were formed by random-dot patterns drifting on a stationary random-dot background. In the absence of drift, the rectangle became invisible. The data obtained showed intriguing news – the absence of overestimation of the relative length of the stimulus in the judgments. Even more, all subjects produced gentle errors of underestimation of the distance. The average value lowered from about zero to -5 arc min with increasing the stimulus length from 25 to 225 arc min. The present result agreed to some extent with previous literature data demonstrating changes in perceived size for moving objects, e.g.: grating motion’s apparent position shifted in the direction of motion, and moving objects seemed smaller. Taken together, the present result showed an exception to the general rule – the contours of motion do not create the illusion of expansion.