Grasping Weber’s law

Ganel et al. [1] recently reported what seem to be fundamentally different effects of varying object size on the precision of different tasks. Whereas the standard deviation in the matched size was larger for larger objects, as predicted by Weber's law, the standard deviation in the maximum grip opening was independent of object size. The authors concluded that visual coding for grasping does not obey Weber's law. We argue that the difference in performance they observed can readily be understood if one considers the sources of information that contribute to each task. Judgments of size should follow Weber's law, but judgments of position should not. Following our earlier suggestion that grasping is based on position information [2], we therefore would not expect grasping to follow Weber's law. We can account for the performance in all three tasks using reasonable values for perceptual precision. We argue therefore that there is no fundamental violation of Weber's law, just an incorrect assumption about the information that is used for grasping. © 2008 Elsevier Ltd. All rights reserved.