Bicycle Illusion – A blurred moving object enhances 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 distant cyclist moving alongside a pair of chains that sag between fence posts. The cyclist appears to bob up and down with the sagging rails. In the lab, the same illusion can be constructed by moving a disc horizontally past a pair of sinusoidal rails.

View these demonstrations at a sufficient distance from the screen that the smallest displays give you a strong bicycle illusion (an illusion of assimilation). Then note what is happening in the two larger displays.

Relative contrast of moving disc and rails (There are three pages in this demonstration. Please click on the screen if you want to advance to the next page before the current one has completed.):

(A) Same contrast disc and rails: When this page is viewed from a distance at which the smallest display gives you a strong bicycle illusion (illusion of assimilation), note that the illusion in the medium-sized display is somewhat ambiguous and that the larger display is showing an illusion of opposition.

(B) Blurred disc: When this page is viewed from the same distance as the last one, the medium display will now also show an illusion of assimilation. And if you look slightly above (or below) the largest display, it too will show you an illusion of assimilation. This means that a less distinct moving object is more prone to the illusion.

(C) Low contrast rails: When this page is viewed from the same distance as the previous two, even the small display does not show a very strong illusion. This means that the illusion depends on the distinct (high contrast) edges of the curved 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.