Measuring motion illusion strength in op art paintings

Using simple black and white patterns, Op artists create vivid sensations of movement which spawned a lively scientific debate about visual processing mechanisms underlying motion illu- sions in static images. Recent studies point in the direction of a rather simple account in terms of involuntary eye movements, which lead to characteristic motion interference patterns (Zanker and Walker, 2004 Naturwissenschaften 91 149 ^ 156). We developed a new method to quantify the strength of the illusion for synthetic `riloids' that were inspired by some of Bridget Riley's paintings, to replace the unreliable rating scale used previously. Observers compared in a 2AFC paradigm the motion sensation for a given test stimulus with that for a set of reference stimuli, which were combinations of a riloid and a checkerboard pattern at different contrast ratios. The dominance of the riloid in the compound pattern led to a strong motion illusion, whereas a dominating checkerboard led to a static percept. The continuous variation of illusion strength between these two extremes is utilised to generate psychometric functions, which allow us to compare the illusion strength for different test stimuli. A pilot study of this method found no clear differences between observers suffering from migraine and controls.