Violations of Weber's law tell us more about methodological challenges in sensorimotor research than about the neural correlates of visual behaviour.

The violation of Weber's law in grasping has been presented as evidence for the claim that grasping is guided by visual information which is distinct from the information used in perceptual tasks. Previously, we contested this claim and argued that biomechanical constraints of the hand might explain why Weber's law cannot be reliably uncovered in grasping movements. In a recent article Manzone and colleagues (2017) show that pantomime grasping follows Weber's law even with objects whose width is close to the hand's biomechanical limit. In this commentary we explain why the biomechanical account does not necessarily predict the violation of Weber's law in a pantomime grasping task and why it seems problematic to use adherence or violation of Weber's law as a criterion to assign tasks to different anatomical pathways.