Formant variability is actively regulated in vowel production

Abstract

Although movement variability is typically attributed to unwanted noise in the sensorimotor system, recent work in reaching has demonstrated that variability may be at least in part actively controlled. However, it is unknown if this is true for speech production, a complex, well-practiced task controlled via non-visual sensory feedback. Here, we test how variability in formant production for vowels may be actively regulated through real-time perturbations to auditory feedback that push vowel formants toward or away from the center of each speaker’s distribution for that vowel in F1/F2 space. These inward and outward perturbations effectively decrease and increase perceived variability, respectively. Participants exposed to the inward-pushing perturbation increased their produced variability both during and after exposure, indicating that lower perceived variability “frees” the motor system to be less precise. However, the outward-pushing perturbation had no consistent effect on produced variability. This suggests that at least some skilled movements are already produced at the lower limits of possible variability. While overall variability did not change, vowel “centering,” a measure of within-trial correction for variability, did increase, suggesting participants became more responsive to errors. Together, these results suggest that variability in speech production is actively regulated.