Hand transport violates Weber's law - but only with visual feedback and hand relative coding

According to a recent report, the visual coding of size for grasping does not obey Weber’s law (Ganel et al, 2008 Current Biology 18 R599–R601). This surprising result has been interpreted as evidence for a fundamental difference between vision-for-perception, which needs to compress a wide range of physical objects to a restricted range of percepts, and vision-for-action as applied to the much narrower range of graspable and reachable objects. To further test this interpretation we studied the precision of hand transport using aiming tasks that varied in the degree of hand-relative vs object-relative visual coding,in the availability of visual feedback during the action, and in the involvement of memory. Speed-precision tradeoffs (Schmidt et al, 1997 Psychological Review 47 451–451) were controlled. Results indicate that Weber’s law holds in all conditions except when the task enforces hand-relative coding and includes visual feedback during the action. These findings suggest caution before we can conclude that actions violate fundamental psychophysical principles.