The Bicycle Illusion: Sidewalk Science Informs the Integration of Motion and Shape Perception

Michael E.J. Masson, Michael D. Dodd, James T. Enns

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


The authors describe a new visual illusion first discovered in a natural setting. A cyclist riding beside a pair of sagging chains that connect fence posts appears to move up and down with the chains. In this illusion, a static shape (the chains) affects the perception of a moving shape (the bicycle), and this influence involves assimilation (averaging) rather than opposition (differentiation). These features distinguish the illusion from illusions of motion capture and induced motion. The authors take this bicycle illusion into the laboratory and report 4 findings: Naïve viewers experience the illusion when discriminating horizontal from sinusoidal motion of a disc in the context of stationary curved lines; the illusion shifts from motion assimilation to motion opposition as the visual size of the display is increased; the assimilation and opposition illusions are dissociated by variations in luminance contrast of the stationary lines and the moving disc; and the illusion does not occur when simply comparing two stationary objects at different locations along the curved lines. The bicycle illusion provides a unique opportunity for studying the interactions between shape and motion perception.

Original languageEnglish (US)
Pages (from-to)133-145
Number of pages13
JournalJournal of Experimental Psychology: Human Perception and Performance
Issue number1
StatePublished - Feb 2009
Externally publishedYes


  • motion and shape perception
  • motion assimilation
  • visual illusion

ASJC Scopus subject areas

  • Experimental and Cognitive Psychology
  • Arts and Humanities (miscellaneous)
  • Behavioral Neuroscience

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