Coarticulation occurs in running speech when one speech sound or phoneme overlaps with another. It can be considered a result of the way we sequence and organize our articulators to efficiently produce consecutive consonants and vowels in fluent speech. Previous research has suggested that measures of coarticulation can provide insight into the maturity of the motor speech planning system (Barbier, Perrier, Ménard, Payan, Tiede, & Perkell, 2013; Zharkova & Hewlett, 2009; Zharkova, Hewlett, & Hardcastle, 2011). Speech stability has also been suggested as an indicator of motor speech maturity in previous research using ultrasound imaging of velar-vowel targets (Frisch, Maxfield, & Belmont, 2016). This study extends research by Frisch, Maxfield, & Belmont (2016) to investigate patterns of velar-vowel coarticulation and speech stability in bilingual children who wear cochlear implants. Ultrasound and acoustic data were recorded from one English-Spanish bilingual participant (P1) who wears bilateral cochlear implants, one English-Spanish bilingual control child (P2) with no hearing impairment, and one English-Spanish bilingual adult speaker. Measures of velar-vowel coarticulation and speech stability across three productions of English and Spanish words were recorded and analyzed following procedures of Wodzinski and Frisch (2006). The participants were asked to produce three repetitions of fifteen English and fifteen Spanish target words starting with a /k/+ vowel sequence. Ultrasound imaging was used to record and trace tongue movement at the point of maximum velar closure. Data was compared between English and Spanish words, across participants, and between repetitions of the same word. In comparing English and Spanish words, child participants (P1 and P2) demonstrated increased coarticulation during Spanish productions. All participants showed decreased stability in Spanish productions when compared to English. Adult participant (P3) showed greater overall stability in productions and consistent coarticulation across both languages. Measures of coarticulation and overall stability were relatively equal across P1 and P2, while P3 showed greater and more stable coarticulation across both languages. Preliminary results support findings in previous research suggesting that anticipatory coarticulation and speech stability could be used as an index for assessing speech motor planning in bilingual and clinical populations (Barbier, Perrier, Ménard, Payan, Tiede, & Perkell, 2013; Frisch, Allen, Betancourt, & Maxfield, 2016; Frisch, Maxfield, & Belmont, 2016; Frisch & Wodzinski, 2014; Zharkova & Hewlett, 2009; Zharkova, Hewlett, & Hardcastle, 2011). Results additionally indicate that a young cochlear implant user who receives early intervention and is learning two languages can develop commensurate motor speech planning systems to that of a typical bilingual peer and that patterns of coarticulation and stability may be different in English and Spanish contexts.

This study uses ultrasound to image onset velar stop consonant articulation in words. By examining tongue body placement, the extent of velar closure variation across vowel contexts provides for the measurement of anticipatory coarticulation while productions within the same vowel context provide measurement of extent of token-to-token variation. Articulate Assistant Advanced 2.0 software was used to semi-automatically generate midsagittal tongue contours at the initial point of maximum velar closure and was used to fit each contour to a curved spline. Patterns of lingual coarticulation and measures of speech motor stability, based on curve-to-curve distance (Zharkova, Hewlett, & Hardcastle, 2011), are investigated to compare the speech of typically fluent speakers to the speech of people who stutter. Anticipatory coarticulation can be interpreted as a quantitative measure indicating the maturity of the speech motor system and its planning abilities. Token-to-token variability is examined from multiple velar vowel productions within the same vowel context, describing the accuracy of control, or stability, of velar closure gestures. Measures for both speaking groups are examined across the lifespan at stages during speech development, maturation, and aging. Results indicate an overall age effect, interpreted as refinement, with increased speech stability and progressively more segmental (less coarticulated) productions across the lifespan. A tendency toward decreased stability and more coarticulated speech was found for younger people who stutter, but this difference was small and absent among older adults. Outcomes of this study suggest the articulatory maturation trajectories of people who stutter may be delayed, but overall maturation of the speech mechanism is evident by older adulthood for typically fluent speakers and those who stutter. Applications to intervention are discussed in closing.