The kinetic Orbison illusion.

In the typical Orbison illusion, the sides of a square placed on concentric circles appear to be bending toward the center of the circles. We report a motion version of the Orbison illusion (namely, the kinetic Orbison illusion). When a dot moves along a square trajectory against a background of concentric circles, the sides of the trajectory appear to bend toward the center and the corners appear to be sharpened. In the present study, observers adjusted the shape of a comparison stimulus to the shape of the perceived trajectory by bending the sides. The amount of illusion was operationally defined as the largest discrepancy between the square and adjusted shape in the comparison stimulus. It was found that the illusory bending was more than twice the static Orbison illusion and reached a maximum of 7.3% of the length of one side. Experiments including a comparison between fixation and pursuit conditions revealed that the main cause of the kinetic illusion was not motion streaks of the dot crossing background circles. We propose an alternative hypothesis based on induced motion generated by background circle motion, the direction of which is misperceived owing to the aperture problem.