Abstract.
Fraser and Wilcox (Nature 1979) devised a series of complex, stationary patterns that provoke episodes of compelling illusory motion, but only in about two-thirds of people tested. Using simplified versions of their stimuli, we have confirmed their claim of perceptual dimorphism.

We show that

• the strength of the illusory motion depends upon stimulus duration, eccentricity, and contrast
• illusory motion does not depend upon fluctuations in accommodation, as has been suggested for some other forms of illusory motion.

Finally, we consider the relation of Fraser-type motion to other forms of illusory motion.