Does the brain see double? The effect of visual illusions on perception and action

Perceiving object size is a complex challenge that the brain often resolves through inferences shaped by visual context. This process is revealed in size‑contrast illusions, which provide a powerful tool for probing how context influences both perception and action. Seminal studies suggest that such illusions do not affect grip aperture during object grasping, supporting the view that perception and action rely on distinct mechanisms for estimating size. However, I will demonstrate that size‑contrast illusions can bias action when opportunities for online correction are limited—for example, in prospective action judgments and saccadic eye movements. These findings challenge the long‑standing assumption that action is immune to contextual distortions. Moreover, I will show that the influence of illusions on saccades diminishes with increased processing time, just as for perceptual judgements, suggesting that perception and action draw on a shared representation that is initially context‑sensitive but becomes more accurate over time. Together, this work shows that perception and action are deeply intertwined, and that sensitivity to context may represent an adaptive feature of human behavior rather than a flaw.