Guttural consonants (i.e., uvular, pharyngealized and pharyngeal) in Arabic are argued to form a natural class due to phonological patterning and use of a common oro-sensory zone in the pharynx (McCarthy, 1994; Sylak-Glassman, 2014a, 2014b). Yet, phonetic studies have failed to successfully find a single phonetic exponent to explain this patterning. In fact, many studies have tried to quantify this patterning by looking at changes within the root of the tongue. In this study, I Generalized Additive Mixed Modelling to quantify the whole tongue changes as obtained from Ultrasound Tongue Imaging. Using various quantification methods (2D and 3D difference splines), I show how gutturals use a common area in the vocal tract, which is indeed located at the tongue root, but also located at the tongue dorsum and body. The observed patterns point towards a gradient rather than categorical change. The phonological feature that can explain these patterns is predominantly the feature [+retracted], which is a subcomponent of the feature [+constricted epilaryngeal tube] (following the predictions of the Laryngeal Articulator Model” (LAM, Esling, 2005; Esling et al., 2019). However, tongue root, dorsum and body changes cannot be simply quantified by the feature [+retracted]. I discuss implications for an alternative formal account.

References
Esling, J. (2005). There Are No Back Vowels: The Laryngeal Articulator Model. The Canadian Journal of Linguistics, 50(1), 13–44. https://doi.org/10.1353/cjl.2007.0007
Esling, J., Moisik, S., Benner, A., & Crevier-Buchman, L. (2019). Voice Quality: The Laryngeal Articulator Model (Issue 1). Cambridge University Press. https://doi.org/10.1017/9781108696555
McCarthy, J. J. (1994). The phonetics and phonology of Semitic pharyngeals. In P. A. Keating (Ed.), Phonological Structure and Phonetic Form (pp. 191–233). Cambridge University Press. https://doi.org/10.1017/CBO9780511659461.012
Sylak-Glassman, J. (2014a). An Emergent Approach to the Guttural Natural Class. Proceedings of the Annual Meetings on Phonology, 1–12. https://doi.org/10.3765/amp.v1i1.44
Sylak-Glassman, J. (2014b). Deriving Natural Classes: The Phonology and Typology of Post-Velar Consonants. University of California, Berkeley.