Movement variability can be modulated in speech production

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.