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The Journal of Neurophysiology Vol. 84 No. 6 December 2000, pp. 2984-2997
Copyright ©2000 by the American Physiological Society
Section for Physiology, Department of Integrative Medical Biology, Umeå University, SE-90187 Umeå, Sweden
Jenmalm, Per,
Seth Dahlstedt, and
Roland
S. Johansson.
Visual and Tactile Information About Object-Curvature Control
Fingertip Forces and Grasp Kinematics in Human Dexterous Manipulation. J. Neurophysiol. 84: 2984-2997, 2000. Most objects that we manipulate have curved surfaces. We have analyzed
how subjects during a prototypical manipulatory task use visual and
tactile sensory information for adapting fingertip actions to changes
in object curvature. Subjects grasped an elongated object at one end
using a precision grip and lifted it while instructed to keep it level.
The principal load of the grasp was tangential torque due to the
location of the center of mass of the object in relation to the
horizontal grip axis joining the centers of the opposing grasp
surfaces. The curvature strongly influenced the grip forces required to
prevent rotational slips. Likewise the curvature influenced the
rotational yield of the grasp that developed under the tangential
torque load due to the viscoelastic properties of the fingertip pulps.
Subjects scaled the grip forces parametrically with object curvature
for grasp stability. Moreover in a curvature-dependent manner, subjects
twisted the grasp around the grip axis by a radial flexion of the wrist
to keep the desired object orientation despite the rotational yield. To
adapt these fingertip actions to object curvature, subjects could use
both vision and tactile sensibility integrated with predictive control. During combined blindfolding and digital anesthesia, however, the
motor output failed to predict the consequences of the prevailing curvature. Subjects used vision to identify the curvature for efficient
feedforward retrieval of grip force requirements before executing the
motor commands. Digital anesthesia caused little impairment of grip
force control when subjects had vision available, but the adaptation of
the twist became delayed. Visual cues about the form of the grasp
surface obtained before contact was used to scale the grip force,
whereas the scaling of the twist depended on visual cues related to
object movement. Thus subjects apparently relied on different
visuomotor mechanisms for adaptation of grip force and grasp
kinematics. In contrast, blindfolded subjects used tactile cues about
the prevailing curvature obtained after contact with the object for
feedforward adaptation of both grip force and twist. We conclude that
humans use both vision and tactile sensibility for feedforward
parametric adaptation of grip forces and grasp kinematics to object
curvature. Normal control of the twist action, however, requires
digital afferent input, and different visuomotor mechanisms support the
control of the grasp twist and the grip force. This differential use of
vision may have a bearing to the two-stream model of human visual processing.
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