Manual asymmetries in bimanual prehension tasks: manipulation of object size and object distance.

Two experiments were designed to investigate the temporal and spatial couplings of the transport and grasp components for bimanual movements to both congruent and incongruent targets. We studied conditions where task requirements were largely different for the two hands. Ten participants performed Experiment 1 and were required to reach for, grasp, and lift two small (1 mm) cylinders, two large (70 mm) cylinders, or one small and one large cylinder with the right and left hands. In Experiment 2, 10 participants were required to reach for, grasp, and lift two objects that were positioned either near (50mm) the start mark, far (maximum comfortable reaching distance) from the start mark, or one near and one far from the start mark. Kinematic measures, relative timing differences between the hands and spatial plots were used to quantify both temporal and spatial couplings of the limbs. For temporal coupling, the results from both experiments indicated that the upper limbs were controlled independently with some execution-level interference occurring for the transport component only. In terms of spatial coupling our results indicated weak coupling of the grasp component regardless of task parameters (i.e., congruent or incongruent movements) and a dependence on task parameters in determining the level of spatial coupling for the transport component. These results can be collectively interpreted as evidence for a functional coupling of the upper limbs. That is, the movements of the hands may be coupled during tasks in which temporal and spatial synchronizations are beneficial for performance. However, if the coupling of the upper limbs is either unimportant or perhaps even detrimental to the coordination of the overall movement, then the upper limbs may perform the desired movements independently of one another.