Reaching and grasping actions and their context shape the perception of object size.

Humans frequently estimate the size of objects to grasp them. In fact, when performing an action, our perception is focused towards the visual properties of the object that enable us to successfully execute the action. However, the motor system is also able to influence perception, but only a few studies have reported evidence for action-induced visual perception modifications. Here, we aimed to look for a feature-specific perceptual modulation before and after a reaching or a grasping action. Human participants were instructed to either reach for or grasp two-dimensional bars of different size and to perform a size perceptual task before and after the action in two contexts: in one where they knew the subsequent type of movement and in the other where they did not know. We found significant modifications of perceived size of stimuli more pronounced after grasping than after reaching. The mere knowledge of the subsequent action type significantly affected the size perception before the movement execution, with consistent results in both manual and verbal reports. These data represent direct evidence that, in natural conditions without manipulation of visual information, the action type and the action context dynamically modulate size perception, by shaping it according to relevant information required to recognize and interact with objects.