Visual Speech Perception Cues Constrain Patterns of Articulatory Variation and Sound Change

Abstract

What are the factors that contribute to (or inhibit) diachronic sound change? While acoustically motivated sound changes are well-documented, research on the articulatory and audiovisual-perceptual aspects of sound change is limited. This paper investigates the interaction of articulatory variation and audiovisual speech perception in the Northern Cities Vowel Shift (NCVS), a pattern of sound change observed in the Great Lakes region of the United States. We focus specifically on the maintenance of the contrast between the vowels /ɑ/ and /ɔ/, both of which are fronted as a result of the NCVS. We present results from two experiments designed to test how the NCVS is produced and perceived. In the first experiment, we present data from an articulatory and acoustic analysis of the production of fronted /ɑ/ and /ɔ/. We find that some speakers distinguish /ɔ/ from /ɑ/ with a combination of both tongue position and lip rounding, while others do so using either tongue position or lip rounding alone. For speakers who distinguish /ɔ/ from /ɑ/ along only one articulatory dimension, /ɑ/ and /ɔ/ are acoustically more similar than for speakers who produce multiple articulatory distinctions. While all three groups of speakers maintain some degree of acoustic contrast between the vowels, the question is raised as to whether these articulatory strategies differ in their perceptibility. In the perception experiment, we test the hypothesis that visual speech cues play a role in maintaining contrast between the two sounds. The results of this experiment suggest that articulatory configurations in which /ɔ/ is produced with unround lips are perceptually weaker than those in which /ɔ/ is produced with rounding, even though these configurations result in acoustically similar output. We argue that these findings have implications for theories of sound change and variation in at least two respects: (1) visual cues can shape phonological systems through misperception-based sound change, and (2) phonological systems may be optimized not only for auditory but also for visual perceptibility.

Articulatory and acoustic dynamics of fronted back vowels in American English

“…McGuire and Babel (2012) argue that visual cues contribute to perceptual asymmetry for /f/ and /h/, while King and Chitoran (2022) argue that the distinct visual quality of British labiodental /r/ (i.e., [V]) helps to distinguish this variant from /w/. Most relevant to the present study, Havenhill and Do (2018) argue that visual perceptibility of lip rounding has contributed to the preservation of rounding on /O/ as it undergoes fronting as part of the Northern Cities Shift (NCS). In contrast to the fronting of /u/, fronting of /O/ in the NCS is not driven by coarticulation, but by pressure to maintain dispersion within the vowel space (Labov, 1994).…”

Difficult to hear but easy to see: Audio-visual perception of the /r/-/w/ contrast in Anglo-English

“…Directly after stimulus presentation, participants identified the word-initial consonant they perceived by clicking on a word from two options using a wireless optical mouse. A 2 000 ms time limit was imposed on responses, after which the program automatically advanced to the next stimulus (as in Havenhill and Do, 2018). Subjects were instructed that their first mouse click would be recorded and were asked to respond as quickly and accurately as possible.…”

“…and [n], may maximize the perceptual distinctiveness of these sounds, which may explain why they occur so frequently cross-linguistically, despite their relatively similar acoustic cues (Dohen, 2009). In a production and audio-visual perception study of the American English /O/-/A/ contrast, which is currently undergoing a merger in some dialects, it was found that a visual labial contrast was retained, despite the merging acoustics (Havenhill and Do, 2018). The authors themselves argued that visual cues may play a role in the shaping of phonological systems by inhibiting misperception of the speech signal in cases where two sounds are acoustically similar.…”

Loose lips and tongue tips: The central role of the /r/-typical labial gesture in Anglo-English

“…Indeed, speech has been shown to be visually optimised in cases where pressure to maintain a phonological contrast is high. For example, Havenhill & Do (2018) observed that in American English, the visual lip rounding cue enhances perception of the /A/-/O/ contrast, and Traunmüller & Öhrström (2007) found that in Swedish, listeners rely on visual cues in the perception of /i/-/y/. Future research could consider whether the different visual cues for /r/ and /w/ are perceptibly salient to Anglo-English speakers.…”