Effect of Ebbinghaus illusion on perceptual and grasping judgements

The theory of two visual systems assumes that visual perception relies on a ventral pathway extending from the striate cortex to the inferior temporal cortex, whereas visually-guided action relies on a dorsal pathway extending from the striate cortex to the superior parietal cortex. The finding that size-contrast illusions deceive perception but not action has long been taken as supporting the independence of these two pathways. For example, the Ebbinghaus illusion shows that the size of a central circle is perceived as smaller or larger according to the size of surrounding circles. However, it is still unclear whether action resists this illusion because of the difficulty to match size judgement and real action in terms of visual and proprioceptive feedback. To overcome this problem, we tested the effect of Ebbinghaus illusion on grasping judgements implying a covert activation of the motor programs involved in grasping. The task required participants to decide whether or not they fell able to grasp the central circle of an Ebbinghaus pattern without moving their hands. As a control, we asked other participants to decide whether or not the central circle was smaller than a previously seen circle used as a reference. In both tasks, participants' performance provides strong evidence that the Ebbinghaus illusion affects both perception and action. These results suggest that common processes underlie the computaion of object size irrespective of vision aim, i.e vision for perception or for action.

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Geers, L., Andres, M., & Pesenti, M. (2016). Effect of Ebbinghaus illusion on perceptual and grasping judgements. Annual meeting of the Belgian Association of Psychological Sciences, Antwerp, Belgium. https://hdl.handle.net/2078.5/273930