Representation of curved surfaces in responses of mechanoreceptive afferent fibers innervating the monkey's fingerpad

Abstract

The aim was to elucidate how the population of digital nerve afferents signals information about the shape of objects in contact with the fingerpads during fine manipulations. Responses were recorded from single mechanoreceptive afferent fibers in median nerves of anesthetized monkeys. Seven spherical surfaces were used, varying from a highly curved surface (radius, 1.44 mm; curvature, 694 m-1) to a flat surface (radius, infinity; curvature, 0 m-1). These were applied to the fibers' receptive fields, which were located on the central portion of a fingerpad. When the objects were located at the centers of the receptive fields, the responses of the slowly adapting fibers (SAIs) increased as the curvature of the surface increased and as the contact force increased. All SAIs behaved in the same way, differing only by a scaling factor (the sensitivity of the individual afferent). Responses of the rapidly adapting afferents were small and did not vary systematically with the stimulus parameters, and most Pacinians did not respond at all. Stimuli were applied at different positions in the receptive fields of SAIs to define the response profiles of the afferents (response as a function of position on the fingerpad). All SAIs had similarly shaped profiles for the same surface curvature and the shape differed for different curvatures. These profiles reflected the shape of the stimulus. An increase in contact force scaled these profiles upward. Thus, the population of digital nerve fibers signals unambiguous information about the shape and contact force of curved surfaces contacting the fingerpad.

Visual and Somatosensory Information about Object Shape Control Manipulative Fingertip Forces

“…These afferents offer many types of information that may be of relevance for the control of fingertip forces in manipulation, for instance, information related to frictional slips and creep Srinivasan et al, 1990;Milner et al, 1991), the shape of the contact surface (Goodwin et al, 1995), and contact angle (Goodwin and Morley, 1987), as well as distribution within the contact area of normal and tangential forces Srinivasan et al, 1990;. Thus, information related to object shape in the present study should have been readily available from signals in populations of tactile afferents.…”

Human vs. robotic tactile sensing: Detecting lumps in soft tissue

“…Goodwin et al stimulated the human finger pad with rigid spherical stimuli of varying radii, and showed that slight changes in curvature (especially for stimuli smaller than a single finger pad) can drastically effect the firing rate of SA I mechanoreceptors [12,14] and the perception [13] of the stimulus curvature. Similarly, Lamotte et al showed that peak pressures vary inversely with the object radius, such that stronger responses would be evoked from the smaller lumps (larger curvature) [21].…”

Texture perception through direct and indirect touch: An analysis of perceptual space for tactile textures in two modes of exploration

“…When scanning a texture with a probe, the perception of the contour of the probe, which is in direct contact with the fingers, is likely mediated peripherally by SA1 afferents (Goodwin et al 1995;LaMotte et al 1998). The roughness of the scanned surface, however, seems to be a function of the vibrations elicited in the probe as it is scanned across the surface (see Figure 6, left panel and Figure 11A).…”