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The Journal of Neurophysiology Vol. 86 No. 2 August 2001, pp. 604-615
Copyright ©2001 by the American Physiological Society
Department of Neuroscience, University of Minnesota, Minneapolis, Minnesota 55455
Baud-Bovy, Gabriel and
John F. Soechting.
Two Virtual Fingers in the Control of the Tripod Grasp. J. Neurophysiol. 86: 604-615, 2001. To
investigate the organization of multi-fingered grasping, we asked
subjects to grasp an object using three digits: the thumb, the index
finger, and the middle or ring finger. The object had three coarse flat
contact surfaces, whose locations and orientations were varied
systematically. Subjects were asked to grasp and lift the object and
then to hold it statically. We analyzed the grasp forces in the
horizontal plane that were recorded during the static hold period.
Static equilibrium requires that the forces exerted by the three digits
intersect at a common point, the force focus. The directions of the
forces exerted by the two fingers opposing the thumb depended on the
orientation of the contact surfaces of both fingers but not on the
orientation of the contact surface of the thumb. The direction of the
thumb's force did not depend on the orientation of the contact
surfaces of the two fingers and depended only weakly on the orientation
of the thumb's contact surface. In general, the thumb's force was
directed to a point midway between the two fingers. The results are
consistent with a hierarchical model of the control of a tripod grasp.
At the first level, an opposition space is created between the thumb and a virtual finger located approximately midway between the two
actual fingers. The directions of the forces exerted by the two fingers
are constrained to be mirror symmetric about the opposition axis. The
actual directions of finger force are elaborated at the next level on
the basis of stability considerations.
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