How preparation to touch or grasp alters visual size perception.

Prior studies have suggested that visually guided actions are resistant to the effects of some pictorial size illusions, e.g., the maximum grip aperture component of a grasp for an element of the Ebbinghaus illusion display. We present evidence that when participants prepare to grasp, the reduction in illusion magnitude observed for action components is also present for conscious perceptual judgments. Our studies characterize how visual size perception changes when we choose to engage in different size-mediated behaviors. Even when the stimuli used were identical for two different tasks, we found that available information was processed differently. In the studies, participants always selected which of the two targets was larger. In some conditions, the context in which the targets were presented induced a visual illusion of size. We varied the sizes of target pairs to assess the magnitude of these visual illusions. In some tasks, participants indicated their size choice verbally. For other tasks, participants reached to grasp or touch the target that they perceived as larger. Illusion magnitudes were smaller when participants engaged in actions directed at a target or when participants imagined performing those actions. This shift in visual processing persisted for several minutes after participants switched back to a verbal, non-grasping, non-touch task. A motor interference task eliminated the reduction in illusion magnitude.