Object motor representation and reaching-grasping control.

The following two competing hypotheses were tested in the present study. Is grasp guided by multiple representations of a single object, each of which codes a different grasp motor act according to the physical properties of that item? Conversely, is grasp guided by a single representation that codes all the possible affordances enabled by the object? Subjects reached different objects, but the object part used by subjects to grasp them was identical. In experiments 1 and 2, two familiar objects (fruits) which varied for size and shape were presented. Subjects grasped their stalks whose size and shape were equal. In experiments 3-7 the presented objects were geometrical solids, which varied, respectively, for weight, volume, intrinsic height, centre of mass and shape. Nevertheless, in all experiments the object portion where subjects' fingers grasped it had the same physical features. Finally, experiment 8 was a control experiment in which subjects reached and grasped equal handles of bells of the same shape, but different size. Volume, shape, and familiarity of the object influenced the grasp kinematics, even if the features of the grasped object part did not change. Variation in intrinsic object height and weight influenced final reach kinematics. Variation in centre of mass influenced neither grasp nor reach kinematics. Data are discussed in support of the hypothesis that a single object motor representation, which codes all the object affordances, is involved in grasp kinematic implementation.