Covert contrast in velar fronting: An acoustic and ultrasound study

ABSTRACT

There is growing evidence that speech sound acquisition is a gradual process, with instrumental measures frequently revealing covert contrast in errors perceived to involve phonemic substitution. Ultrasound imaging has the potential to expand our understanding of covert contrast by showing whether a child uses different tongue shapes while producing sounds that are perceived as neutralised. This study used an ultrasound measure (Dorsum Excursion Index) and acoustic measures (VOT and spectral moments of the burst) to investigate overt and covert contrast between velar and alveolar stops in child speech. Participants were two children who produced a perceptually overt velar-alveolar contrast and two children who neutralised the contrast via velar fronting. Both acoustic and ultrasound measures revealed significant differences between perceptually distinct velar and alveolar targets. One child with velar fronting demonstrated covert contrast in one acoustic and one ultrasound measure; the other showed no evidence of contrast. Clinical implications are discussed in this article.

KEYWORDS:

• Acoustics
• child speech
• covert contrasts
• ultrasound
• velars

Acknowledgements

This paper benefited from contributions by the following student assistants: Reim Farag, Rachel Kadison, Emily Lerner, and Michelle Marron. We thank Doug Whalen for helpful input on the design of the study. Finally, we are grateful to our participants and their families for their cooperation throughout the study. Aspects of this research were presented at the annual convention of the American Speech-Language Hearing Association in Orlando (2014).

Funding

This project was supported by an NYU Steinhardt IDEA grant and by NIH R03DC 012883.

Declaration of interest

The authors report no conflicts of interest.

Notes

¹ Characterisation of a contrast as “covert” does not entail that it is impossible to perceive. A sufficiently fine-grained perceptual task, such as narrow transcription by a trained listener, or quantification of gradient degrees of perceived difference using visual analog scaling (Munson, Schellinger, & Urberg-Carlson, 2012) can succeed in identifying a contrast that is not represented in broad transcription. Throughout this paper, contrasts will be characterised as “neutralised” if they would not be distinguished by a casual listener or in broad transcription; it remains possible that an expert listener or fine-grained perceptual scale could tease out a contrast.

² Because stabilisation is never perfect, it is often paired with video recording or optical tracking to detect changes in probe placement, which can be used to correct or discard problematic trials (Whalen et al., 2005). These techniques were not used in the present study.

³ This helmet was developed by the authors in collaboration with a mechanical engineer. It is subjectively judged to assist with probe stabilisation, particularly to minimise rotational changes in probe angle that can alter the plane of section. However, formal testing of this device has not been undertaken.

⁴ The ultrasound machine was in the room with the microphone and thus contributed to background noise, but its contribution was consistent across all recordings. As this study focuses on within-subject comparisons, no corrections for this consistent background noise were undertaken.

⁵ Inclusion of the perceptually accurate velar targets did not affect the significance of the comparison for Max’s session 2 (t = −0.57, df = 20.9, p = 0.57).

⁶ Note that this is the only analysis in which data from Emma and Jayden, the two children who produced a perceptually overt contrast, were merged. This was for reader convenience: because the four spectral moments were examined in separate regressions, the number of results reported quickly becomes overwhelming. When regressions for these two individuals were computed separately, velar versus alveolar target place remained a robustly significant predictor of all four spectral moments for both children.

⁷ The spectral moments of Max’s perceptually accurate velars were consistent with expectations from the children who produced a typical contrast, with the one exception that accurate velars produced in Session 1 were very similar to fronted velars and alveolars with respect to the second spectral moment, slope.

Additional information

Funding

This project was supported by an NYU Steinhardt IDEA grant and by NIH R03DC 012883.