Interference between normal vibrato and artificial stimulation of laryngeal muscles at near-vibrato rates.

A stabilized tremor hypothesis for vocal vibrato is investigated. The stabilizer is assumed to be a mechanical oscillator that may contain reflex loops. Artificial stimulation of the cricothyroid muscle in one subject showed a well-defined resonance curve of this peripheral oscillator at approximately 5.0 Hz. Combined artificial stimulation with natural vibrato showed that the vibrato could be entrained by a peripheral stimulus, provided the two frequencies are separated by no more than approximately +/- 0.5 Hz. This suggests that vibrato frequencies are not "hard-wired" centrally, even though a collection of centrally generated tremors may serve as excitation to the peripheral oscillator.