Illusory size interacts with current hand posture during prospective action judgements

How does the eye guide the hand in an ever-changing world? The two visual system model proposes that visually-guided actions require the computation of the absolute metrics of an object, with respect to the observer and without being influenced by the visual context. Accordingly, actions should be immune to visual illusions arising from a size contrast between a target and context elements. However, experimental studies revealed discrepant results that remain difficult to reconcile. Proprioceptive and visual information of the acting hand have been identified as potential confounds since they may be exploited to correct the illusory effect. To overcome the issue, we investigated the effect of a visual illusion in which the relative size of a circle varies according to the size of surrounding circles (i.e., The Ebbinghaus illusion) on prospective action judgements. Participants had to decide whether they thought to be able to grasp the central circle of an Ebbinghaus display between their index finger and thumb without moving their hands. A control group had to judge the size of the central circle relative to a standard. Experiment 1 showed that the illusion affected perceptual and grasping judgements similarly. We investigated further the interaction between the visual illusion and grip aperture representation by examining the effect of concurrent motor tasks on grasping judgements. We showed that participants underestimated their grasping ability when they were squeezing a ball between their index finger and thumb (Experiment 2), whereas they overestimated their ability when their fingers were spread apart (Experiment 3). The illusion modulated the interference of the squeezing movement with the illusion of largeness enhancing the underestimation of one’s grasping ability observed in Experiment 2. We concluded that both the visual context and the body posture influence action anticipation, which is therefore not totally immune to optical illusions.

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Geers, L., Pesenti, M., & Andres, M. (2018). Illusory size interacts with current hand posture during prospective action judgements. Symposium “Vision by interaction” at the Annual meeting of the Belgian Association of Psychological Sciences, Gent. https://hdl.handle.net/2078.5/28756