Biomechanical Simulation of Lateral Asymmetry in Tongue Bracing

Abstract

Tongue bracing occurs when the lateral edges of the tongue maintain contact with the palate or upper molars. Bracing is pervasive in speech: previous EPG data analysis shows lateral contact to be maintained for 97.5% of the total observation duration [Gick et al. (2017) JSLHR 60, 494]. This research has also indicated the presence of lateral bias or asymmetry in the tongue: a consistent tendency to release contact more on one side than the other, both in unilateral release and in sequential loss of contact during a bilateral release. However, in these cases of asymmetry, it remains unclear which side of the tongue should be considered dominant. The present study explores the question of dominance in lateral tongue bracing through biomechanical simulation of tongue muscle activation. Certain muscles have been identified as bracing agonists (GGP, GGM, MH, VERT, SL) and antagonists (GGA, STY, HG, TRANS, IL) [Liu et al. (2022) Phonetica 79, 523]. Agonists increase the likelihood of bilateral bracing and tend to raise or widen the tongue while antagonists decrease the likelihood of bilateral bracing and tend to lower or narrow the tongue. The current simulations explore whether a unilateral bracing outcome can be produced by asymmetric activation of agonists, antagonists or a combination thereof. Preliminary results from our simulation study show that reducing agonist activation or increasing antagonist activation on one side while keeping activations constant on the other side results in lower tongue-palate contact on the opposite side, due to the hydrostatic properties of the tongue, though the reduction is much greater in the former case. These results suggest that unilateral bracing may be instantiated primarily by contra-lateral muscle activation/deactivation from the side on which bracing contact is maintained, providing a starting point for further exploration of lateral tongue dominance.