Vibrotactile atonal interval correlated with activity in peripheral mechanoreceptive units innervating the human hand

The quality of sensations and the vibrotactile atonal interval-the gap between detection and vibration thresholds-were studied with vibratory stimuli of varying frequency (20, 80 and 160 Hz) and duration (100, 400 and 800 ms) applied to the hairy and glabrous skin of the hand. Detection and vibration thresholds were also determined while simultaneously recording single unit activity from the radial nerve innervating the hairy skin of the hand. Both thresholds were lower on the glabrous than the hairy skin, and the thresholds decreased on both skin areas with increasing vibration frequency. A sensation of short duration was elicited at detection threshold only with a 20-Hz stimulus of 100-ms duration; with other frequency-duration combinations sensations of longer duration were reported. Considerably larger vibration amplitudes were needed on both skin areas for the sensations to be unequivocal with respect to duration and pitch (vibration threshold). There was no significant effect of stimulus duration on vibrotactile thresholds. The width of the average atonal intervals was above 10 dB on both skin areas, and with increasing vibration frequency, decreasing values of atonal intervals were obtained on the hairy skin, whereas considerably increasing values were obtained on the glabrous skin. Recording of single unit activity indicates that on the hairy skin detection of the stimulus at 20 Hz is correlated with activation of slowly adapting (SA) type II and the most sensitive rapidly adapting (RA) units, while distinct vibratory sensations involve entrainment of RA units. Also at 80 Hz, non-pacinian units could contribute to the mechanism of vibrotactile thresholds, whereas at 160 Hz only pacinian (PC) units are involved.