A new explanation of the Fraser-Wilcox illusion

The Fraser-Wilcox illusion is seen in a circular pattern that is divided into sectors containing repeated luminance gradients. The pattern appears to rotate, especially when the observer makes saccadic eye movements or blinks. The illusion has been known since 1979, but its origin is still disputed. The most prominent theory, which proposes that the latency of neural responses varies across the pattern (Conway et al., J. Neuro., 2005), cannot explain the long duration of the motion or why the illusion is weaker in older participants, or under pinhole viewing. We propose that the illusion is caused by changes in the retinal luminance of the pattern, due either to changes in pupil diameter during eye movements or to the transit of the eyelid across the pupil during blinks. It is already known that luminance change can generate illusory motion in patterns containing luminance gradients (e.g., Mather, Vision Res., 1984), so we argue that the Fraser-Wilcox illusion is part of this illusion class. This new explanation can account for the importance of eye movements and blinks, the effect of age (reduced pupil mobility), and the duration of the motion impressions. A simulation of the illusion in which pattern luminance is modulated with the same time-course as that caused by blinks and saccades creates a markedly enhanced impression of illusory motion.