REPRESENTATION OF CURVED SURFACES IN RESPONSES OF MECHANORECEPTIVE AFFERENT-FIBERS INNERVATING THE MONKEYS FINGERPAD

The aim was to elucidate how the population of digital nerve afferents signals information about the shape of objects in contact with the fi ngerpads during fine manipulations. Responses were recorded from singl e mechanoreceptive afferent fibers in median nerves of anesthetized mo nkeys. Seven spherical surfaces were used, varying from a highly curve d surface (radius, 1.44 mm; curvature, 694 m(-1)) to a flat surface (r adius, infinity; curvature, 0 m(-1)). These were applied to the fibers ' receptive fields, which were located on the central portion of a fin gerpad. When the objects were located at the centers of the receptive fields, the responses of the slowly adapting fibers (SAls) increased a s the curvature of the surface increased and as the contact force incr eased. All SAls behaved in the same way, differing only by a scaling f actor (the sensitivity of the individual afferent). Responses of the r apidly adapting afferents were small and did not vary systematically w ith the stimulus parameters, and most Pacinians did not respond at all . Stimuli were applied at different positions in the receptive fields of SAls to define the response profiles of the afferents (response as a function of position on the fingerpad). All SAls had similarly shape d profiles for the same surface curvature and the shape differed for d ifferent curvatures. These profiles reflected the shape of the stimulu s. An increase in contact force scaled these profiles upward. Thus, th e population of digital nerve fibers signals unambiguous information a bout the shape and contact force of curved surfaces contacting the fin gerpad.