Encoding of object curvature by tactile afferents from human fingers. A.W. Goodwin, V.G. Macefield & J.W. Bisley. Dept of Anatomy & Cell Biology, University of Melbourne, Parkville, Victoria 3052, Australia; and Prince of Wales Medical Research Institute, Randwick, NSW 2031, Australia.
APStracts 4:191N, 1997.
ABSTRACT
Isolated responses were recorded from fibers in the median nerves of human subjects using microneurography. Mechanoreceptive afferent fibers with receptive fields on the fingerpads were selected. The fingers were immobilized, and spherical stimuli were applied passively to the receptive field with a contact force of 40, 60 or 80 g wt. The radii of the spheres were 1.92, 2.94, 5.81, 12.4 mm or _ (flat); the corresponding curvatures, given by the reciprocal of the radii, were 694, 340, 172, 80.6 or 0 m-1 respectively. When the spheres were applied to the receptive field center of slowly adapting type I afferents (SAIs), the response increased as the curvature of the sphere increased and also increased as the contact force increased. All SAIs behaved in the same way except for a scaling factor proportional to the sensitivity of the afferent. When a sphere was located at different positions in the receptive field, the shape of the resulting response profile reflected the shape of the sphere; for more curved spheres the profile was higher and narrower (increased peak and decreased width). Slowly adapting type II afferents (SAIIs) showed different response characteristics from the SAIs when spheres were applied to their receptive field centers. As the curvature of the stimulus increased from 80.6 to 172 m-1 the response increased, but further increases in curvature did not result in further increases in response. An increase in contact force resulted in an increase in the response of SAIIs; this increase was proportionately greater than it was for SAIs. For SAIIs the shape of the receptive field profile did not change when the curvature of the stimulus changed. For fast adapting type I afferents (FAIs), the responses were small and did not change systematically with changes in curvature or contact force. Fast adapting type II afferents (FAIIs) did not respond to our stimuli. Human SAIs, FAIs and FAIIs behaved like monkey SAIs, FAIs and FAIIs respectively. The response of the SAI population contains accurate information about the shape of the sphere and its position of contact on the finger, and also indicates contact force. Conversely, while SAIIs possess a greater capacity to encode changes in contact force, they provide only coarse information on local shape. 

Received 3 June 1997; accepted in final form 12 August 1997.
APS Manuscript Number J458-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 28 August 1997