Time course of the effect of the Muller-Lyer illusion on saccades and perceptual judgments.

The amplitude of saccadic eye movements is affected by size illusions such as the Müller-Lyer illusion, but this effect varies highly between studies. Here we examine the origin of this variability by testing the influence of three temporal factors on the effect of the Müller-Lyer illusion: presentation time, response delay, and saccade latency. Subjects performed reflexive saccades, deferred saccades, and memory-guided saccades along the shaft of the illusion. We evaluated the time course of the saccadic illusion effects. We compared it to the influence of presentation time on the illusion effect in a perceptual judgment task. According to the "two visual systems hypothesis", visual perception and visual memory rely on a perceptual representation coded along the ventral "perception" pathway, which is affected by visual contextual illusions. Visuomotor actions, such as saccades, depend on the dorsal "action" pathway that is largely immune to illusions. In contrast with this hypothesis, our results show that the illusion affected both saccade amplitude and perceptual judgments with a similar time course. Presentation time of the Müller-Lyer illusion, not response delay or saccade latency, was the major factor in determining the size of the illusion effect. Longer presentation times resulted in smaller effects, suggesting that our visual representation is dynamic and becomes more accurate when we look at an object for a longer time before we act on it.