Common organization for unimanual and bimanual reach-to-grasp tasks.

In two experiments comparisons between characteristics of performance of a unimanual and a bimanual reach-to-grasp (prehension) task were made on an individual subject basis. The unimanual prehension task used required that the object be grasped by finger and thumb pad opposition, the bimanual task required that the grasp be made by opposing the pads on the two index fingers. Experiment 1 examined adaptation of prehension movements to objects of different size (width) but equal grasp surface area placed at different distances. Experiment 2 examined adaptation of movements to objects of different grasp surface areas. It was found that the aperture and transport components of the two prehension tasks developed over time in very similar fashion in all subjects. Movements were adapted to different task constraints in the same way as has previously been reported in the literature and were very similar in both tasks: maximum aperture increases with increasing object size and occurs later in the movement for larger objects; movement time increases with target distance; time of maximum aperture occurs earlier in the movement for targets with smaller grasp surface areas; movement times are longer for such objects, largely due to increases in the deceleration phase of the movement. These results support the notion that there is an effector independent level of organization that governs the coordination of movements during performance of reaching and grasping tasks.