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Increased speech contrast induced by sensorimotor adaptation to a non-uniform auditory perturbation.

When auditory feedback is perturbed in a consistent way, speakers learn to adjust their speech to compensate, a process known as sensorimotor adaptation. While this paradigm has been highly informative for our understanding of the role of sensory feedback in speech motor control, its ability to induce behaviorally-relevant changes in speech that affect communication effectiveness remains unclear. Because reduced vowel contrast contributes to intelligibility deficits in many neurogenic speech disorders, we examine human speakers' ability to adapt to a non-uniform perturbation field which was designed to affect vowel distinctiveness, applying a shift that depended on the vowel being produced. Twenty-five participants were exposed to this "vowel centralization" feedback perturbation in which the first to formant frequencies were shifted towards the center of each participant's vowel space, making vowels less distinct from one another. Speakers adapted to this non-uniform shift, learning to produce corner vowels with increased vowel space area and vowel contrast to partially overcome the perceived centralization. The increase in vowel contrast occurred without a concomitant increase in duration and persisted after the feedback shift was removed, including after a 10-minute silent period. These findings establish the validity of a sensorimotor adaptation paradigm to increase vowel contrast, showing that complex, non-uniform alterations to sensory feedback can successfully drive changes relevant to intelligible communication.