Auditory-Motor Control of Fundamental Frequency in Vocal Vibrato.

PURPOSE: The purpose of this study was to investigate how classically trained singers use their auditory feedback to control fundamental frequency (fo) during production of vocal vibrato. Two main questions were addressed: (1) Do singers produce reflexive foresponses to sudden perturbation of the fo of their auditory feedback during production of vibrato indicative of feedback control? (2) Do singers produce adaptive foresponses to repeated perturbation of the fo of their auditory feedback during production of vibrato indicative of feedback and feedforward control? In addition, one methodological question was addressed to determine if adaptive fo responses were more precisely assessed with or without an auditory cue for fo during the repeated fo perturbation paradigm. METHOD: Ten classically trained singers produced sustained vowels with vibrato while the fo and harmonics of their auditory feedback were suddenly perturbed by 100 cents to assess reflexive control or repeatedly perturbed by 100 cents to assess adaptive control. Half of the participants completed the repeated perturbation experiment with an auditory cue for fo, and the other half completed the experiment without an auditory cue for fo. Acoustical analyses measured changes in mean fo in response to the auditory feedback perturbations. RESULTS: On average, participants produced compensatory responses to both sudden and repeated perturbation of the fo of their auditory feedback. The magnitude of the responses to repeated perturbations was larger than the responses to sudden perturbations. Responses were also larger in the cued, repeated fo perturbation experiment than in the uncued, repeated fo perturbation experiment. CONCLUSIONS: These findings indicate that classically-trained singers use both feedforward and feedback mechanisms to control their average fo during production of vibrato. When compared to prior studies of singers producing a steady voice, the reflexive fo responses were larger in the current study, which may indicate that the feedback control system is engaged more during production of vibrato.

The Effects of Masked and Delayed Auditory Feedback on Fundamental Frequency Modulation in Vocal Vibrato.

PURPOSE: Although relatively precise control over the extent and rate of fundamental frequency (fo) modulation may be needed for optimal production of vibrato, the role of auditory feedback in controlling vibrato is not well understood. Previous studies altered the gain and timing of auditory feedback in singers producing vibrato and showed inconsistent effects on the extent and rate of fo modulation, which may have been related to small sample sizes or limited analyses. Therefore, the purpose of this study was to further investigate whether the gain or timing of auditory feedback impacts control of vibrato in a larger sample of speakers and with advanced statistical analyses. METHOD: Ten classically-trained singers produced sustained vowels with vibrato while their auditory feedback was masked with pink noise or multi-talker babble to reduce the gain of their auditory feedback and while their auditory feedback was delayed by about 200 or 300 milliseconds to alter the timing of their auditory feedback. Acoustical analyses measured changes in the extent and rate of fo modulation in the masked and delayed trials relative to control trials. Bayesian modeling was used to analyze the effects of noise-masked, babble-masked, and delayed auditory feedback on the extent and rate of fo modulation. RESULTS: There was compelling evidence that noise masking increased the extent of fo modulation, and babble masking increased the variability in the rate of fo modulation (ie, jitter of fo modulation). Masked auditory feedback did not affect the average rate of fo modulation. Delayed auditory feedback did not affect the extent, rate, or jitter of fo modulation. CONCLUSIONS: The current study demonstrated that reducing the gain of the auditory feedback with noise masking increased the extent of fo modulation but did not affect the average rate of fo modulation in classically-trained singers producing vibrato. Reducing the gain of the auditory feedback with babble masking and altering the timing of auditory feedback with imposed delays did not affect the average extent or rate of fo modulation. However, babble masking increased the jitter of fo modulation rate, which suggests that modulated auditory feedback may affect the periodicity of fo modulation from one modulation cycle to the next. These findings clarify the role of auditory feedback in controlling vibrato and may inform the current reflex-resonance models of vibrato.