Illusory Object Motion on Stripes

A circular object placed in the center of a radial pattern consisting of thin sectors causes a robust motion illusion during smooth-pursuit eye movement. We tested whether the motion illusion also arose when the radial background was replaced by horizontal or vertical stripes or a plain background in light gray (in the averaged luminance of the striped area). Subjects reported the illusion strength of a comparison stimulus having stripes or a plain background by a magnitude estimation method, comparing it with the standard stimulus having a radial background. The results show that this illusion occurred when the direction of eye movement and the orientation of the stripes were orthogonal or the yellow circular object was equiluminant with the surroundings. In the tested range, the finer the stripes were, the stronger the illusions were. However, the illusion caused by the stripes was much weaker than that caused by the radial pattern. With the plain light-gray background, the illusion was very weak. Thus, these factors alone cannot fully explain the strong illusory effect of the radial background.