Planning and control of hand orientation in grasping movements.

Humans grasp objects in a way that facilitates the intended use of the object. We examined how humans grasp a circular control knob in order to turn it in different directions and by different extents. To examine the processes involved in anticipatory planning of grasps, we manipulated advance information about the location of the control knob and the target of the knob-turn. The forearm orientation at the time of grasping depended strongly on the knob-turn, with the direction of the knob-turn having a stronger effect than the extent of the knob-turn. However, the variability of the forearm orientations after the knob-turn remained considerable. Anticipatory forearm orientations began early during the grasping movement. Advance information had no influence on the trajectory of the grasp but affected reaction times and the duration of the grasp. From the results, we conclude that (1) grasps are selected in anticipation of the upcoming knob rotation, (2) the desired hand location and forearm orientation at the time of grasping are specified before the onset of the grasp, and (3) an online programming strategy is used to schedule the preparation of the knob-turn during the execution of the grasp.