Visual coding for action violates fundamental psychophysical principles

According to Weber's law, a basic perceptual principle of psychological science, sensitivity to changes along a given physical dimension decreases when stimulus intensity increases [1]. In other words, the 'just noticeable difference' (JND) for weaker stimuli is smaller - hence resolution power is greater - than that for stronger stimuli on the same sensory continuum. Although Weber's law characterizes human perception for virtually all sensory dimensions, including visual length [2,3], there have been no attempts to test its validity for visually guided action. For this purpose, we asked participants to either grasp or make perceptual size estimations for real objects varying in length. A striking dissociation was found between grasping and perceptual estimations: in the perceptual conditions, JND increased with physical size in accord with Weber's law; but in the grasping condition, JND was unaffected by the same variation in size of the referent objects. Therefore, Weber's law was violated for visually guided action, but not for perceptual estimations. These findings document a fundamental difference in the way that object size is computed for action and for perception and suggest that the visual coding for action is based on absolute metrics even at a very basic level of processing.