Slowly adapting type I afferents from the sides and end of the finger respond to stimuli on the center of the fingerpad.

The central part of the fingerpad in anesthetized monkeys was stimulated by spheres varying in curvature indented into the skin. Responses were recorded from single slowly adapting type I primary afferent fibers (SAIs) innervating the sides and end of the distal segment of the stimulated finger. Although these afferents had receptive field centers that were remote from the stimulus, their responses were substantial. Increasing the curvature of the stimulus resulted in an increased response for most afferents. In general, responses increased most between stimuli with curvatures of 0 (flat) and 80.6 m(-1), with further increases in curvature having progressively smaller effects on the response. We calculated an index of sensitivity to changes in curvature; this index varied widely among the afferents but for most it was less than the index calculated for afferents innervating the fingerpad in the vicinity of the stimulus. Responses of all the SAIs increased when the contact force of the stimulus increased. An index of sensitivity to changes in contact force varied widely among the afferents but in all cases was greater than the index calculated for SAIs from the fingerpad itself. Neither the curvature sensitivity nor the force sensitivity of an afferent was related in any obvious way to the location of its receptive field center on the digit. There was only a minor correspondence between an afferent's sensitivity to force and its sensitivity to curvature. The large number of afferents innervating the border regions of the digit do respond to stimuli contacting the central fingerpad; they convey some information about the curvature of the stimulus and substantial information about contact force.