Grasping an object naturally or with a tool: are these tasks guided by a common motor representation?

The aim of the present study was to determine whether kinematic parameters of the grasping motor act are controlled independently of the biomechanical features of the grasping effector. With this purpose in mind, we compared grasping movements performed naturally or using a tool. The tool consisted of two mechanical fingers whose opening and closing phases required squeezing (flexion of the biological fingers) and releasing (extension of the biological fingers) of a handle, respectively. The forces required for opening and closing the mechanical fingers were, respectively, greater and smaller than those used to grasp the objects naturally. In a control experiment the participants grasped with their thumb and index finger the same objects grasped with the tool. The kinematics of the mechanical and biological fingers as well as those of the arm in the two experiments were compared with each other. Grasping an object with the tool showed some kinematic characteristics strikingly similar to those of the natural grasp, whereas others were different. Like the natural grasp, the tool grasp consisted of a finger opening and closing phase. The scaling of both peak velocity of aperture and maximal aperture of the mechanical fingers as a function of object size was the same as that of the biological fingers. In contrast, the tool grasp differed from the natural one for the temporal aspects of the movement. Finally, the initial reach (i.e. the acceleration phase) was poorly influenced by the tool use whereas the final reach (i.e. the deceleration phase) was lengthened and more sensitive to object size. We discuss the results of the present study as being in favour of the hypothesis that some features of the grasp motor representation are coded in cortical areas independently of the used effector. In addition, they suggest a partial independence between the reach and the grasp components.