Semantic grasping escapes Weber's law

Abstract

According to Weber's law, the just noticeable difference between stimuli increases proportionally with stimulus magnitude, suggesting that perception becomes more variable when a stimulus becomes larger. Surprisingly, this basic psychophysical principle appears to be violated in grasping because the variability of grasping movements does not increase with object size. This dissociation between perception and grasping has been interpreted either as evidence for different neuronal processing of real-time visual size information [Ganel, T., Chajut, E., Algom, D. (2008a). Current Biology, 18(14), R599–R601], or for the idea that grasping ignores stimulus size and is based on position information only [Smeets, J. B. J., and Brenner, E. (2008). Current Biology, 18(23), R1089–R1090]. Both accounts assume that it is the processing of visual information that leads to the absence of Weber's law in grasping. We show that even if neither visual nor any real-time sensory information about the stimulus is presented (but only abstract, semantic information about its size), grasping does not follow Weber's law. This indicates that other mechanisms must be responsible for the unexpected behavior of grasping.