The perception and neural coding of complex vibrotactile stimuli

The perception and neural coding of AM and FM modulated, sinusoidal vibrations were investigated in a series of studies that combined psychophysical experiments on humans with neurophysiological experiments on peripheral mechanoreceptive afferents in monkeys. The stimuli consisted of pure and modulated vibratory stimuli delivered by a cylinder and a 1-mm diameter punctate probe. Each stimulus type was vibration in directions parallel and perpendicular to the skin.; The psychophysical studies showed: (1) when subjects actively grasped the cylinder, absolute detection thresholds for 200 Hz stimuli were as low as 0.02 {dollar}mu{dollar}m. (2) Thresholds were highly affected by changes in contact area but were unaffected by changes in contact force and the direction of vibration. (3) At low modulation frequencies {dollar}rm (fsb{lcub}m{rcub}s),{dollar} subjects perceived the modulation envelope, whereas at high {dollar}rm fsb{lcub}m{rcub}s{dollar} the percept was more complex, being described by some subjects as rough. (4) Modulation detection thresholds varied with the waveform type, the center frequency {dollar}rm (fsb{lcub}c{rcub}){dollar} and the modulation frequency.; The neurophysiological study using the cylinder showed: (1) While both RA and PC afferents responded to pure tone and modulating stimuli at 40 Hz, the PC afferents responded with much higher firing rates than RA afferents. (2) Only the PC afferents responded to pure tone and modulating stimuli with a 300 Hz {dollar}rm fsb{lcub}c{rcub}.{dollar} (3) SA afferents did not respond to vibratory stimuli delivered by the cylinder, at either frequency. (4) For RA and PC afferents, the mean firing rate across the population did not change with modulation depth; however, both the instantaneous rate and phase-locking responses increased with modulation level.; A comparison of the psychophysical and the neurophysiological data indicates that at 40 Hz modulation detection is coded by modulation in the instantaneous rate and phase-locking responses, in predominantly the PC afferent population. This coding scheme does not account for the psychophysical data obtained at 300 Hz.