Title: Movement variability can be modulated in speech production
Legend: Experiment design. A: schematic (top) and examples from representative participants (bottom) of perturbations applied to speech vowel formants during the exposure phase: inward pushing (experiment 1, left), outward pushing (experiment 2, center), and large outward pushing (experiment 3, right). Note different axis scales in the example data across experiments. Black circles represent the formant values participants produced, and colored circles indicate what was played back to them over headphones. The ellipses represent a 95% confidence interval around the data points of the same color. Note that all formant frequency alterations were applied in mels, a logarithmic measure of frequency. B: experimental procedure and magnitudes of the perturbation applied in experiments 1–3 (left) and experiment 4 (right).
Citation: Tang, D. L., Parrell, B., & Niziolek, C. A. (2022). Movement variability can be modulated in speech production. Journal of neurophysiology, 128(6), 1469–1482. https://doi.org/10.1152/jn.00095.2022
Abstract: Although movement variability is often attributed to unwanted noise in the motor system, recent work has demonstrated that variability may be actively controlled. To date, research on regulation of motor variability has relied on relatively simple, laboratory-specific reaching tasks. It is not clear how these results translate to complex, well-practiced tasks. Here, we test how variability is regulated during speech production, a complex, highly overpracticed, and natural motor behavior that relies on auditory and somatosensory feedback. Specifically, in a series of four experiments, we assessed the effects of auditory feedback manipulations that modulate perceived speech variability, shifting every production either toward (inward pushing) or away from (outward pushing) the center of the distribution for each vowel. Participants exposed to the inward-pushing perturbation (experiment 1) increased produced variability while the perturbation was applied as well as after it was removed. Unexpectedly, the outward-pushing perturbation (experiment 2) also increased produced variability during exposure, but variability returned to near-baseline levels when the perturbation was removed. Outward-pushing perturbations failed to reduce participants’ produced variability both with larger perturbation magnitude (experiment 3) and after their variability had increased above baseline levels as a result of the inward-pushing perturbation (experiment 4). Simulations of the applied perturbations using a state-space model of motor behavior suggest that the increases in produced variability in response to the two types of perturbations may arise through distinct mechanisms. Together, these results suggest that motor variability is actively monitored and can be modulated even in complex and well-practiced behaviors such as speech. NEW & NOTEWORTHY: By implementing a novel auditory feedback perturbation that modulates participants’ perceived trial-to-trial variability without affecting their overall mean behavior, we show that variability in the speech motor system can be modulated. By assaying speech production, we expand our current understanding of variability to a well-practiced, complex behavior outside of the limb control system. Our results additionally highlight the need to incorporate the active control of variability in models of speech motor control.
About the Lab: The Niziolek lab studies how the feedback system functions in persons with aphasia, many of whom have deficits in speech production and error awareness. They are developing laboratory studies of speech skill learning that can be translated to training interventions for speakers such as these who have trouble consistently producing auditory targets. The lab’s proposed training consists of vocal games that map speech to a real-time visual display. These visual speech training games have the potential to be adapted into tools to improve speech production in individuals with speech impairment, including deaf speakers and children with developmental speech and language disorders.
Investigator: Caroline A Niziolek, PhD
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