The Journal of Neuroscience, January 1, 1998, 18(1):521-530
Peripheral Neural Mechanisms Determining the Orientation of
Cylinders Grasped by the Digits
M. J.
Dodson1,
A. W.
Goodwin1,
A. S.
Browning2, and
H. M.
Gehring1
1 Department of Anatomy and Cell Biology, University of
Melbourne, Parkville, Victoria 3052, Australia, and
2 Department of Experimental Psychology, Oxford University,
Oxford OX1 3UD, England
When a human grasps a cylindrical object, feedback on the
orientation of the cylinder with respect to the axes of the digits is
crucial for successful manipulation of the object. We measured the
ability of humans to discriminate the orientations of cylinders passively contacting the fingerpad. For a cylinder of curvature of 521 m
1 (radius, 1.92 mm) subjects were able to
discriminate, at the 75% level, orientation differences of 5.4°; for
a less curved cylinder (curvature, 172 m1; radius,
5.81 mm) the difference limen decreased to 4.2°. The neural
mechanisms underlying the determination of tactile orientation were
investigated by recording the responses of single slowly adapting type
I afferents (SAIs) innervating the fingerpads of anesthetized monkeys.
When cylinders were stepped across the receptive field of an SAI, the
resulting response profiles were Gaussian in shape; the shape
corresponded to the shape of the cylinder, increasing in height and
decreasing in width for more curved cylinders. All SAIs had the same
underlying profile shape except for a multiplicative constant
determined by the sensitivity of the individual afferent. Thus it was
possible to reconstruct the response of the population of active SAIs
in the fingerpad. Changing the orientation of the cylinder resulted in
a rotation of the population response, but the change in angle of the
population response was greater than the change in orientation of the
cylinder. This discrepancy increased as the orientation of the cylinder
moved closer to the orientation of the axis of the finger and was more
pronounced for the less curved cylinder. Measured contact areas between
the cylinders and the skin were elliptical, with orientations exceeding
those of the cylinder; again the differences were greater for the less curved cylinder and for orientations closer to that of the finger axis.
The human discrimination performance could be explained in terms of the
SAI population responses.
Key words:
tactile orientation; mechanoreceptor; cutaneous afferent; finger; grasp; shape; curvature; cylinder; skin mechanics
Copyright © 1998 Society for Neuroscience 0270-6474/98/181521-10$05.00/0
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