Abstract
According to Weber's law, a fundamental principle of perception, sensitivity to changes in magnitude along a given physical dimension decreases when stimulus magnitude increases. In other words, the increment needed in order to detect a change - the Just Noticeable Difference (JND) - is smaller for weak stimuli compared to stronger stimuli. Although Weber's law governs human perception for visual dimensions, including visual length, there have been no attempts to test its validity for visually-guided action rather than for perception. Based on the notion that visually-guided action and perception are mediated by distinct neuroanatomical systems, we hypothesized that Weber's law does not necessarily hold for visually-guided action. In order to test this idea, we asked observers to either grasp or make perceptual estimations of length for objects varying in length. These measurements served to assess resolution with these two functions of vision. We found that the JND increased with object size for perceptual estimations in accordance with Weber's law. For grasping, however, the JND remained invariant across different sizes of objects, violating Weber's law. In a series of follow-up experiments, we tested the presence of Weber's law in online grasping as compared with that in memory-based grasping as well as under open-loop conditions in which vision was occluded during the grasping movement. Collectively, our findings reinforce the presence of a basic dissociation between the way object size is computed for perception and for action.