Independent movements of the digits in grasping
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Reaching out for an object is often considered to consist of the control of two components: transporting the hand to the object's position, and scaling the grip to the object's size. We recently proposed an alternative view. According to this view, grasping consists of controlling the digits, not the hypothetical transport and grip. This alternative view assumes that the opening of the hand emerges from the trajectories of the digits. We therefore studied the movements of the digits in grasping. We asked subjects to grasp disks (diameters ranging from 5 to 8 cm) at marked positions with two digits. The positions were at opposite sides of the disk, at the same distance from the starting position, so that the orientation of the surface was the same for both digits. The subjects grasped the disks either with the index finger and thumb of the dominant hand, with the same digits of the non-dominant hand, or bimanually with both index fingers. Our predictions are: that the well-known relation between object size and grip aperture holds for each digit; that the same relation holds if the object is grasped with two hands instead of with the thumb and finger of one hand; that maximum deviation, variability and duration of the digit movements are related; and that variations in the timing of the maximum deviation of one digit are independent of those in the other digit. In accordance with our predictions, we found that the maximum deviation of both digits increased with 0.75 times the object radius, independent of the hand(s) used. The movements of the thumb were more variable than those of the index finger, which was reflected by a larger deviation earlier in the movement. The timing of the maximum deviation of the two digits was independent. These results on the digits' movements are consistent with our view that grasping can be understood as the largely independent movements of the digits. The results are not in conflict with the hypothesis that the grip is controlled during grasping, but can only be explained by extending that hypothesis post hoc.
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