ENCODING OF OBJECT CURVATURE BY TACTILE AFFERENTS FROM HUMAN FINGERS

Isolated responses were recorded from fibers in the median nerves of h uman subjects by using microneurography. Mechanoreceptive afferent fib ers 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 weight. The radii of the spheres were 1.92, 2.94, 5.81, or 12.4 mm or infinity (ha t); 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 spher e increased and also increased as the contact force increased. All SAI s behaved in the same way except for a scaling factor proportional to the sensitivity of the afferent. When a sphere was located at differen t positions in the receptive field, the shape of the resulting respons e profile reflected the shape of the sphere; for more curved spheres t he 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 rece ptive field centers. As the curvature of the stimulus increased from 8 0.6 to 172 m(-1) the response increased. However, further increases in curvature did not result in further increases in response. An increas e in contact force resulted in an increase in the response of SAIIs; t his increase was proportionately greater than it was for SAIs. For SAI Is, the shape of the receptive held profile did not change when the cu rvature of the stimulus changed. For fast-adapting type I afferents (F AIs), 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 b ehaved like monkey SAIs, FAIs, and FAIIs, respectively. The response o f the SAI population contains accurate information about the shape of the sphere and its position of contact on the finger and also indicate s contact force. Conversely, whereas SAIIs possess a greater capacity to encode changes in contact force, they provide only coarse informati on on local shape.