When a black or white rectangle drifts horizontally across a background of black and white vertical stripes, the rectangle appears to stop and start as it crosses each stripe (the footsteps illusion; Anstis, 2001 Perception 30 785–794). Although previous studies indicate that confusion between contrast and velocity signals in the motion detectors or the spatial pattern of the background contribute to the footsteps illusion (e.g., Sunaga et al., 2008 Perception 37 902–914), it remains unclear which factor is critical. We hypothesize that the contour of the rectangle is significant to the footsteps illusion. A subjective experiment is conducted using modified rectangles, the contour of which were emphasized by adding contour lines, filling random dots inside, or putting illusory contour inducers on the four corners. Two kinds of rectangles were presented above and below central fixation simultaneously and the background strips were scrolled from right to left, or vice versa. Participants were asked which rectangle was perceived to drift more smoothly. The results demonstrate that the footsteps illusion is reduced when the rectangle's contour is emphasized. Placing random dots inside the rectangle yielded a weaker illusion than the rectangle that was surrounded by lines. These results suggest that humans perceive the velocity of moving objects (or background) based on the extracted contours which are constructed by integrating low spatial frequency information.
