The time-dependent modulation of saccade amplitude by illusory length reflects a shared representation between perception and action.

Geers, Laurie;Doré-Mazars, Karine;Andres, Michael
(2025) Journal of Experimental Psychology: Human Perception and Performance — Vol. Advance online publication, n° 10.1037/xhp0001382, p. 1-36 (2025)

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Abstract
Are eye movements miscalibrated when the distance to be crossed is misperceived? The two-visual-stream hypothesis posits that while perception is influenced by visual context and thus prone to illusions, actions rely on context-independent metrics and are thus unaffected by such distortions. In contrast, empirical evidence consistently shows that saccadic eye movements are influenced by the Müller-Lyer illusion. However, this finding could be explained by saccades being attracted toward the figure's center of gravity, while perceptual distortion would result from distinct mechanisms. To disentangle these effects, we conducted four experiments (N = 114) between 2022 and 2025, measuring the amplitude of saccades performed along horizontal lines embedded in Müller-Lyer figures carefully designed to control for center-of-gravity biases. Results showed that both saccade amplitude and length judgment were systematically modulated by the illusion beyond what could be attributed to changes in the actual center of gravity, its saliency, or computation time. Additionally, the illusion's influence on saccade amplitude diminished after a longer previewing time (2,000 ms) compared to a shorter one (50 ms). These findings support the hypothesis that perception and oculomotor behavior rely on a shared visual representation that is influenced by contextual information but becomes more precise with increased processing time.
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Geers, L., Doré-Mazars, K., & Andres, M. (2025). The time-dependent modulation of saccade amplitude by illusory length reflects a shared representation between perception and action. Journal of Experimental Psychology: Human Perception and Performance, Advance online publication(10.1037/xhp0001382), 1-36. https://doi.org/10.1037/xhp0001382 (Original work published 2025)