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

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.