Increased vowel contrast and intelligibility in connected speech induced by sensorimotor adaptation.

Abstract Alterations to sensory feedback can drive robust adaptive changes to the production of consonants and vowels, but these changes often have no behavioral relevance or benefit to communication (e.g., making “head” more like “had”). This work aims to align the outcomes of adaptation with changes known to increase speech intelligibility – specifically, adaptations that increase the acoustic contrast between vowels in running speech. To this end, we implemented a vowel centralization feedback perturbation paradigm that pushes all vowels towards the center of vowel space, making them sound less distinct from one another. Speakers across the adult lifespan adapted to the centralization perturbation during sentence production, increasing the global acoustic contrast among vowels and the articulatory excursions for individual vowels. These changes persisted after the perturbation was removed, including after a silent delay, and showed robust transfer to words that were not present in the sentences. Control analyses demonstrated that these effects were unlikely to be due to explicit pronunciation strategies and occurred in the face of increasingly more rapid and less distinct production of familiar sentences. Finally, sentence transcription by crowd-sourced listeners showed that speakers’ vowel contrast predicted their baseline intelligibility and that experimentally-induced increases in contrast predicted intelligibility gains. These findings establish the validity of a sensorimotor adaptation paradigm to implicitly increase vowel contrast and intelligibility in connected speech, an outcome that has the potential to enhance rehabilitation in individuals who present with a reduced vowel space due to motor speech disorders, such as the hypokinetic dysarthria associated with Parkinson’s disease. Significance statement During speech production, humans learn from perceived errors by adjusting their speech to compensate, a process known as sensorimotor adaptation. Here we leveraged sensorimotor adaptation to cause an increase in the distinctiveness of speech sounds. Specifically, speakers rapidly and implicitly learned vowel-specific articulation changes during sentence production, with positive effects on their intelligibility. This study is important for models of sensorimotor integration across all motor domains, as it demonstrates the simultaneous learning of multiple opposing transformations in the course of continuous movement. Our findings show that sensorimotor adaptation can be successfully applied to drive changes relevant to a vital ecological behavior: intelligible communication.