Action is immune to the effects of Weber's law throughout the entire grasping trajectory

Vision for action and vision for perception have been suggested to be mediated by overlapping yet dissociable mechanisms. Recent evidence provided basic psychophysical support for this suggestion. In particular, it has been shown that Weber's law, the decrease in visual resolution with object size, does not affect the maximum grip aperture (MGA) between the fingers prior to grasp. Several studies replicated this result, but have argued that grasping may still obey Weber's law at early movement stages. Here, we show that this apparent adherence to Weber's law was confounded by task demands. Specifically, subjects were asked to keep their fingers closed prior to grasp, which encouraged them to open their fingers faster for larger compared to smaller objects. In two experiments, we tested this proposal by disentangling the effects of velocity from those of Weber's law. In Experiment 1, subjects were asked to keep their fingers open wide-apart prior to grasp, which encouraged them to close their fingers faster, rather than slower, to smaller objects. Now, the apparent adherence to Weber's law was reversed, and higher resolution was found for larger compared to smaller objects, thus indicating a "reversed" Weber's law. In Experiment 2, we manipulated task demands to equate aperture velocities across different objects sizes. When velocity was equated, no effects of Weber's law were found throughout the movement. These findings show that previous studies have confounded visual resolution with fingers' velocity, which led to an erroneous conclusion that Weber's law affected grasping at early stages of the movement.