RECEPTOR ENCODING OF MOVING TACTILE STIMULI IN HUMANS .1. TEMPORAL PATTERN OF DISCHARGE OF INDIVIDUAL LOW-THRESHOLD MECHANORECEPTORS

The response of 70 cutaneous, low-threshold mechanoreceptors in the hu man median, radial and inferior alveolar nerves to well controlled bru sh stimuli moving across the receptive field was quantitatively studie d. Microneurography was used to obtain the response of each to multipl e velocities from 0.5 to 32 cm/sec in at least two opposing directions . A high degree of response consistency was observed from the slowly a dapting receptors to replication of the same stimulus and to a lesser, but significant degree from the fast adapting receptors. The evoked d ischarge reflected up to three partially overlapping phases of the mov ing stimulus: skin compression, indentation, and stretch. Although the overall discharge rate increased with both stimulus velocity and forc e, the spatial discharge pattern was preserved to a high degree. In co ntrast, the discharge patterns differed for opposing and orthogonal di rections. Reducing the area of skin surrounding the receptive field th at was contacted by the moving stimuli had little effect on the evoked response. Individual mechanoreceptors display highly reliable differe nces to brush stimuli moving at different velocities. Moreover, differ ent directions of movement evoke differences in the discharge that are consistently observed upon replication of the same stimuli. Despite t he richness and consistency in the spatial discharge pattern displayed by individual receptors, it is argued that the details of the pattern s are not likely used by the CNS to infer information about direction and velocity of movement across the skin. Rather, the intensity of dis charge is proposed as a plausible information-bearing attribute of the stimulus-evoked response.