Articulatory effort modulates speech adaptation to auditory perturbations

Speech sensorimotor adaptation is typically partial, varies across individuals, and often saturates under large auditory perturbations. While sensory and phonological factors have been proposed to explain this variability, the role of motor effort and its influence on the compensatory response remain largely unexplored. Our study examined whether the physical effort involved in producing compensatory gestures influences both the overall magnitude of adaptation and the perturbation level at which this adaptation begins to saturate. Native French speakers produced the minimal pair /ne/-/nø/ while receiving gradually upshifted second-formant (F2) feedback (0-50%) in two conditions using deformable lip tubes: a very flexible tube (FLT) and a more rigid tube (RLT), both allowing lip rounding but increasing the effort required to achieve it. Compensation was quantified acoustically (F2 decrease) and physiologically (increase in EMG activity of the orbicularis oris muscle) in 21 participants. F2 compensation increased with perturbation level but saturated earlier and at a lower magnitude when articulation required greater effort (RLT). Lip-muscle activity showed a similar nonlinear pattern, with saturation in EMG and F2 occurring at comparable perturbation levels. Variations in F1 suggested that most participants initially relied on increased lip rounding, but some shifted at higher perturbation levels toward another articulatory strategy very likely to involve tongue backing. These results show that speech adaptation depends not only on error monitoring and correction but is also constrained by the physical effort required to produce compensatory movements, which both limits the magnitude of adaptation and influences the selection of compensatory strategies.