Discrimination and sensorimotor adaptation of self-produced vowels in cochlear implant users.

Humans rely on auditory feedback to monitor and adjust their speech for clarity. Cochlear implants (CIs) have helped over a million people restore access to auditory feedback, which significantly improves speech production. However, there is substantial variability in outcomes. This study investigates the extent to which CI users can use their auditory feedback to detect self-produced sensory errors and make adjustments to their speech, given the coarse spectral resolution provided by their implants. First, we used an auditory discrimination task to assess the sensitivity of CI users to small differences in formant frequencies of their self-produced vowels. Then, CI users produced words with altered auditory feedback in order to assess sensorimotor adaptation to auditory error. Almost half of the CI users tested can detect small, within-channel differences in their self-produced vowels, and they can utilize this auditory feedback towards speech adaptation. An acoustic hearing control group showed better sensitivity to the shifts in vowels, even in CI-simulated speech, and elicited more robust speech adaptation behavior than the CI users. Nevertheless, this study confirms that CI users can compensate for sensory errors in their speech and supports the idea that sensitivity to these errors may relate to variability in production.

Palate shape influence depends on the segment: Articulatory and acoustic variability in American English /ɹ/ and /s/.

This ultrasound and acoustics study of American English /ɹ/ and /s/ investigates whether variability in production as measured in the midsagittal plane is related to individual differences in the shape of the hard palate in the coronal plane. Both token-to-token variability and variability between different phonetic contexts were investigated. While no direct relationship was found between palate flatness and articulatory variability, a secondary analysis revealed that speakers' articulatory variability for one segment was related to their variability in the other. Speakers with flatter palates tended towards lower articulatory variability scores, but speakers with more domed palates showed both high and low variability scores.

Effects of syllable stress in adaptation to altered auditory feedback in vowels.

Unstressed syllables in English most commonly contain the vowel quality [ə] (schwa), which is cross-linguistically described as having a variable target. The present study examines whether speakers are sensitive to whether their auditory feedback matches their target when producing unstressed syllables. When speakers hear themselves producing formant-altered speech, they will change their motor plans so that their altered feedback is a better match to the target. If schwa has no target, then feedback mismatches in unstressed syllables may not drive a change in production. In this experiment, participants spoke disyllabic words with initial or final stress where the auditory feedback of F1 was raised (Experiment 1) or lowered (Experiment 2) by 100 mels. Both stressed and unstressed syllables showed adaptive changes in F1. In Experiment 1, initial-stress words showed larger adaptive decreases in F1 than final-stress words, but in Experiment 2, stressed syllables overall showed greater adaptive increases in F1 than unstressed syllables in all words, regardless of which syllable contained the primary stress. These results suggest that speakers are sensitive to feedback mismatches in both stressed and unstressed syllables, but that stress and metrical foot type may mediate the corrective response.

Modeling the effect of palate shape on the articulatory-acoustics mapping.

Articulatory variability is reduced for people with flatter palates [Bakst and Lin (2015). Proceedings of the 18th International Congress of Phonetic Sciences; Brunner, Fuchs, and Perrier (2009). J. Acoust. Soc. Am. 125(6), 3936-3949]. Brunner, Fuchs, and Perrier [(2009). J. Acoust. Soc. Am. 125(6), 3936-3949] hypothesized that this is because the mapping between articulation and acoustics depends on palate depth. Articulatory synthesis was used with three different palate shapes to generate productions of /r/. The parameter spaces of the articulatory synthesizers were searched for vocal tract configurations that result in low F3 (the hallmark acoustic cue for /r/). Palate shape influences not only the sensitivity of the articulatory-acoustic mapping, but also the effect of each individual articulatory parameter on F3.