Independent control of the digits predicts an apparent hierarchy of visuomotor channels in grasping

If an object changes position at the onset of a reach-to-grasp movement, both the transport speed and the grip aperture are adjusted. If the object changes in size at the onset, only the grip aperture is adjusted. This combination of results has been interpreted as being the consequence of a hierarchical relationship between visuomotor channels for transport and grip. We argue that our alternative view on grasping can account for the observed behaviour without making new assumptions. In our view, grasping consists of smooth (minimal jerk) movements of each digit to a target position on the object. The digits' target positions change, both when object position and when object size change. A model in which the individual digits move smoothly to these new positions yields the same behaviour as is observed experimentally.