Perceived pitch of vibrotactile stimuli: effects of vibration amplitude, and implications for vibration frequency coding.

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1. The effect of changes in amplitude on the perceived pitch of cutaneous vibratory stimuli was studied in psychophysical experiments designed to test whether the coding of information about the frequency of the vibration might be based on the ratio of recruitment of the PC (Pacinian corpuscle-associated) and RA (rapidly adapting) classes of tactile sensory fibres. The study was based on previous data which show that at certain vibration frequencies (e.g. 150 Hz) the ratio of recruitment of the PC and RA classes should vary as a function of vibration amplitude. 2. Sinusoidal vibration at either 30 Hz or 150 Hz, and at an amplitude 10 dB above subjective detection thresholds was delivered in a 1 s train to the distal phalangeal pad of the index finger in eight human subjects. This standard vibration was followed after 0.5 s by a 1 s comparison train of vibration which (unknown to the subject) was at the same frequency as the standard but at a range of amplitudes from 2 to 50 dB above the detection threshold. A two-alternative forced-choice procedure was used in which the subject had to indicate whether the comparison stimulus was higher or lower in pitch (frequency) than the standard. 3. Marked differences were seen from subject to subject in the effect of amplitude on perceived pitch at both 30 Hz and 150 Hz. At 150 Hz, five out of the eight subjects reported an increase in pitch as the amplitude of the comparison vibration increased, one experienced no change, and only two experienced the fall in perceived pitch that is predicted if the proposed ratio code contributes to vibrotactile pitch judgements. At 30 Hz similar intersubject variability was seen in the pitch-amplitude functions. 4. The results do not support the hypothesis that a ratio code contributes to vibrotactile pitch perception. We conclude that temporal patterning of impulse activity remains the major candidate code for pitch perception, at least over a substantial part of the vibrotactile frequency bandwidth.

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