Vibrotactile thresholds in mechanoreceptive afferents innervating the foot pad of the cat. The importance of stimulus frequency and duration.

Vibrotactile thresholds were determined for non-Pacinian (SA and RA) and Pacinian (PC) mechanoreceptive units innervating the foot pad of the cat with vibratory stimuli of varying frequency (20, 80, and 240 Hz) and duration (100, 400, and 800 ms). Both the absolute (1 impulse/stimulus train) and the tuning (1 impulse/cycle) thresholds of SA and RA units were elevated and those of PC units were decreased as a function of increasing vibration frequency. PC units had the lowest thresholds at 80 and at 240 Hz whereas RA units had the lowest thresholds at 20 Hz. Larger atonal intervals (the range between the absolute and the tuning thresholds) were obtained for all unit types with increasing vibration frequency and PC units had the smallest atonal intervals at all frequencies. The absolute thresholds of all unit types were independent of stimulus duration. The tuning thresholds of SA and RA units were elevated with increasing stimulus duration whereas the tuning thresholds of PC units were independent of stimulus duration. Consequently, larger atonal intervals were obtained for RA and SA but not for PC units with increasing stimulus duration. The results indicate that there are both differences (effect of frequency) and similarities (effect of stimulus duration and the width of atonal intervals) in response characteristics of non-Pacinian mechanoreceptors innervating the hairy skin and the glabrous foot pad of the cat.