Response: When does grasping escape Weber's law?

In a recent study [1Ganel T. Chajut E. Algom D. Visual coding for action violates fundamental psychophysical principles.Curr. Biol. 2008; 18: R599-R601Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar], we found that Weber's law, a fundamental principle of perception, does not govern visual control of grasping and concluded that different representations of object size are used for action and for perception [1Ganel T. Chajut E. Algom D. Visual coding for action violates fundamental psychophysical principles.Curr. Biol. 2008; 18: R599-R601Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar]. Smeets and Brenner [2Smeets B.J. Brenner E. Grasping Weber's law.Curr. Biol. 2008; 18: R1089-R1090Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar] suggest instead that grasping is computed on the basis of position rather than on the basis of size, and that this accounts for the apparent absence of Weber's law. However, their alternative explanation cannot readily account for memory-based grasping, which does obey Weber's law. In this response, we present additional data to show that, even when memory-based and real-time grasping both are executed without visual feedback, only the former obeys Weber's law. This dissociation further supports the conclusion that action and perception are sustained by qualitatively different computations.