RECEPTOR ENCODING OF MOVING TACTILE STIMULI IN HUMANS .2. THE MEAN RESPONSE OF INDIVIDUAL LOW-THRESHOLD MECHANORECEPTORS TO MOTION ACROSS THE RECEPTIVE-FIELD

The mean firing rate evoked in 70 cutaneous, low-threshold mechanorece ptors in the human median, radial, and inferior alveolar nerves by sti mulus motion across the skin was quantitatively studied. Moving stimul i, controlled far velocity, direction, and length of skin traversed, w ere provided by a servo-controlled motor that carried a brush across t he receptive field. Each unit was studied with stimuli delivered at mu ltiple velocities from 0.5 to 32 cm/sec in at least two opposing direc tions. A power function provided an excellent description of the MFR-v ersus-velocity relationship. The exponent n was interpreted to reflect the receptor's sensitivity to changes in stimulus velocity, and the m ultiplicative constant c,the predicted response to stimuli moving at 1 .0 cm/sec. The fast adapting mechanoreceptors exhibited higher sensiti vity to stimulus velocity than the slowly adapting mechanoreceptors. T he mean velocity at which the fast adapting units were predicted to fi rst respond to movement was also higher. Estimates of n, c, or both di ffered significantly for stimuli delivered in opposing directions for more than 70% of the mechanoreceptors. No direction of motion consiste ntly led to power function parameters with higher values so as to sugg est a ''preferred'' regional direction of motion for the entire popula tion. Neither the directional difference in n nor c could be attribute d to directional differences in the forces applied across the receptiv e fields. These findings suggest that information about velocity and d irection is represented in the mean firing rate responses evoked in th e population of mechanoreceptors activated by a moving tactile stimulu s.