The effect of the Müller-Lyer configuration on saccadic eye movements is not fully due to illusory perception.

Previous research has shown that both perception and oculomotor control are affected by visual illusions. While these findings appear to suggest a common code of visual processing for perception and oculomotor control, there remains the possibility that the perceptual and the oculomotor effects emerge through partially different processes. In three experiments, we replicated the previous finding that perception and saccades were both biased by the typical Müller-Lyer configurations. However, using a non-Müller-Lyer setup in which the perceptual illusion effect was much restrained, we did not observe a comparable reduction in the saccadic effect. Instead, the saccadic effect by Müller-Lyer configuration could be partially due to the center-of-gravity (CoG) effect (i.e., the tendency for saccades to land at the center of gravity of the stimuli). These results indicate that the influence of the Müller-Lyer configuration on saccadic eye movements is a mixed effect of perceptual representation and CoG, rather than exclusively due to the illusory perception. We further found that the saccadic and perceptual effects were not correlated at the trial-by-trial level, which suggest that there could be largely independent sources of noise for perception and saccadic control.NEW & NOTEWORTHY The Müller-Lyer illusion affects both perception and oculomotor control, but it is unknown whether these effects arise from the same or different underlying mechanisms. We developed a modified version of the Müller-Lyer configuration, which largely reduced the perceptual illusion effect compared with the typical configuration but reduced the saccadic effect to a much less extent. Such difference indicates that influence of the Müller-Lyer configuration on saccadic eye movements is not fully mediated by illusory perception.