Bicycle Illusion – Viewing size, not speed, governs the illusion

Bicycle illusion demo to accompany Masson et al (2009)

  • Masson, M.E.J., Dodd, M. D., & Enns, J.T. (in press). The bicycle illusion: Sidewalk science informs the integration of motion and shape perception. Journal of Experimental Psychology: Human Perception & Performance. [pdf]

In nature, the bicycle illusion can be seen when viewing a cyclist moving alongside a pair of chains that sag between fence posts. The cyclist appears to bob up and down with the sagging rails. But, note that you must view the cyclist from some distance to get this effect. If you get too close, a different illusion may be seen–that of the cyclist bobbing in opposition to the sagging rails.

In the lab, this illusion can be constructed by moving a disc horizontally past a pair of sinusoidal rails. In a large display, designed to mimic a near viewing distance, the disc bobs up and down out of step with the rails (an illusion of opposition). In a small display, the disc bobs up and down with the rails (an illusion of assimilation), just like the original bicycle illusion.

Try viewing this display from different distances. If you get far enough away from the screen, even the larger display will reveal the bicycle illusion.

Speed or Size? Two different speeds of motion are shown for each size of rail. The faster disc is moving about three times faster than the slower disc. This means that during a fixed period of time, the slower disc in the smaller display is traveling past the same number of waves as the faster disc in the larger display. This shows that the type of illusion that occurs (opposition versus assimilation) is governed by the size of the display, not by the speed of the disc relative to the number of curves in the rails.

  1. View the demo in your web-browser (as .gif file) Caution: animations shown do not result in smooth motion. For a more accurate demonstration, please view the PowerPoint file that can be downloaded below.
  2. Download demonstration (as .ppt file)

Warning for Mac Users – This .ppt file requires Microsoft PowerPoint 2004. To open the file, hold down the option button and click the link. Save the file and then use Microsoft PowerPoint 2004 to view the presentation.