Abstract
Previous research has suggested that the intention to act can affect the perception of size. Vishton et al. (2007) found that the magnitude of the Ebbinghaus illusion decreases when the perceiver intends to grasp or touch the illusion. These results suggest that merely intending to act on an object, either grasping or touching, can affect the perception of its size. However, it is unclear whether this effect is specific only to the size perception of objects on which an action can be performed. For example, if an object is too large to be grasped, would size perception be affected by the impossible action of grasping? In the present experiment, we addressed this issue by assessing the effects of action intention on the magnitude of the Ebbinghaus illusion in two different displays. In one display, the circles that comprised the illusion were too large to be grasped, and in the other display, the circles were much smaller and capable of being grasped. We found that the intention to act on the large display (non-graspable) did not affect the magnitude of the illusion, while the intention to act on the small display (graspable) decreased the magnitude of the illusion. These results suggest that the effect of action intention on perception is constrained by the capability to perform the intended action. In both display conditions, we found that the illusion magnitude was negatively correlated with the participants' hand lengths but only when they intended to act on the illusion. These results suggest that the perception of the target's size in the Ebbinghaus illusion is scaled with respect to action intention and the observers' bodily capabilities (hand size) for the intended action of grasping.