Visual Estimation of the Size of a Stimulus of Motion-Defined Contour

Abstract

In psychophysical experiments on a monitor screen, a subjective contour of a rectangular shape was formed by a random dot pattern drifting against a background of static random dots. Bidirectional drifting: divergence, and convergence were applied within the rectangle area. The aim was to answer whether the direction of movement selectively influences the perceived relative length of the motion-defined shape. Subjects judged the relative extension of the reference stimulus by adjusting the perceptually equal length of the test gap. It was demonstrated that dots moving horizontally toward the stimulus periphery and tangentially approaching the rectangle edges in the case of divergence caused an overestimation of the rectangle length. The positive errors of the subjects indicated that the rectangle edges perceptually shifted outward. The shift values were about the same for shorter and middle rectangles but lower for longer ones. The size expansion effect appeared to reduce at a certain motion distance. When convergence was tested, dots escaping the rectangle edges and drifting toward the stimulus center caused relatively low error values that continuously declined with stimulus length. There were even negative signs of errors for six subjects out of eight for longer rectangles. The effect of expansion was questionable. Consequently, the approaching and escaping dots don’t act the same way on the positions of the subjective contour. But was the expansion produced by the divergent motion of the same origin as the expansion of stimuli outlined by a static spatial contrast in luminance, color, or texture, and defined by perceptual grouping and the Kanizsa contours (Bielevicius, et al., 2023)? To compare the two manifestations of expansion, the control stimuli were formed of static lines and exposed on the same background of random dots in the present study. The divergently drifting dots were still presented within the rectangle area. The rectangle sizes were the same as before that. Such stimuli caused overestimation errors exceeding those of the motion defined contours. In addition, the expansion strengthened, while not decreasing, with an increase in the length of the stimulus. As a result, for long stimuli, the expansion of the static contour became two to three times higher than that of the motion-defined contour. The size expansion effects for static and motion-defined contours may have different neural origins.