The Journal of Neuroscience, October 15, 2001, 21(20):8222-8237
Encoding of Direction of Fingertip Forces by Human Tactile
Afferents
Ingvars
Birznieks1,
Per
Jenmalm1,
Antony W.
Goodwin2, and
Roland S.
Johansson1
1 Department of Integrative Medical Biology, Physiology
Section, Umeå University, SE-901 87 Umeå, Sweden, and
2 Department of Anatomy and Cell Biology, University of
Melbourne, Victoria, Australia 3010
In most manipulations, we use our fingertips to apply time-varying
forces to the target object in controlled directions. Here we used
microneurography to assess how single tactile afferents encode the
direction of fingertip forces at magnitudes, rates, and directions
comparable to those arising in everyday manipulations. Using a flat
stimulus surface, we applied forces to a standard site on the fingertip
while recording impulse activity in 196 tactile afferents with
receptive fields distributed over the entire terminal phalanx. Forces
were applied in one of five directions: normal force and forces at a
20° angle from the normal in the radial, distal, ulnar, or proximal
directions. Nearly all afferents responded, and the responses in most
slowly adapting (SA)-I, SA-II, and fast adapting (FA)-I afferents were
broadly tuned to a preferred direction of force. Among afferents of
each type, the preferred directions were distributed in all angular
directions with reference to the stimulation site, but not uniformly.
The SA-I population was biased for tangential force components in the
distal direction, the SA-II population was biased in the proximal
direction, and the FA-I population was biased in the proximal and
radial directions. Anisotropic mechanical properties of the fingertip
and the spatial relationship between the receptive field center of the
afferent and the stimulus site appeared to influence the preferred
direction in a manner dependent on afferent type. We conclude that
tactile afferents from the whole terminal phalanx potentially
contribute to the encoding of direction of fingertip forces similar to
those that occur when subjects manipulate objects under natural conditions.
Key words:
microneurography; human hand; cutaneous mechanoreceptors; fingertip force; directional sensitivity; tactile afferents
Copyright © 2001 Society for Neuroscience 0270-6474/01/21208222-16$05.00/0
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