Movements of individual digits in bimanual prehension are coupled into a grasping component

Abstract

The classic understanding of prehension is that of coordinated reaching and grasping. An alternative view is that the grasping in prehension emerges from independently controlled individual digit movements (the double-pointing model). The current study tested this latter model in bimanual prehension: participants had to grasp an object between their two index fingers. Right after the start of the movement, the future end position of one of the digits was perturbed. The perturbations resulted in expected changes in the kinematics of the perturbed digit but also in adjusted kinematics in the unperturbed digit. The latter effects showed up when the end position of the right index finger was perturbed, but not when the end position of the left index finger was perturbed. Because the absence of a coupling between the digits is the core assumption of the double-pointing model, finding any perturbation effects challenges this account of prehension; the double-pointing model predicts that the unperturbed digit would be unaffected by the perturbation. The authors conclude that the movement of the digits in prehension is coupled into a grasping component.

Figure 1. Schematic view of experimental setup and the task.

Participants were to grasp the object between the two index fingers. Right after the hand started moving either the left side or the right side of the target object slid out of a common case (see also Movie S1). See text for details.

Figure 2. Effects of the perturbation on digit trajectories.

Average positions of the two index fingers when unperturbed (solid symbols) and perturbed (open symbols). Panels A and B present the effects of the perturbations as a function of position along the principal reaching direction. Note that the center of the target object was at an x-position of 350 mm. Panels C and D) present the perturbation effects as a function of the moment during reaching. Schematics specify the conditions; Asterisks indicate significant effects of the perturbation (p<.01).