Acoustic Effects of Asymmetric Laryngeal Activation

Objective Symmetric activation of laryngeal intrinsic musculature is considered essential for optimal voice production. However, phonatory consequences of asymmetric laryngeal activation are poorly understood. The objective of this study was to investigate the phonatory characteristics of an asymmetrically activated larynx. Methods Bilateral recurrent laryngeal nerves (RLNs) were stimulated in a graded fashion (from threshold to maximal activation) in a canine phonation model. 121 combinations of left‐right RLN activation states (spanning symmetric and asymmetric activation states) were analyzed for phonatory instabilities (number of bifurcations) in the 1.5s acoustic signal after neuromuscular activation. Results Asymmetric laryngeal activation produced frequent phonatory instabilities. Highest levels of instabilities were seen during phonation at extreme asymmetric conditions and asymmetries during near‐maximum RLN stimulation. Linear regression models demonstrated significant association ( P <. 01) between percent of RLN stimulation and acoustic/aerodynamic measures. Phonation onset time (ms) (Left RLN: Coef = 1.44, CI = 1.01‐1.86; Right RLN: Coef = 2.71, CI = 2.29‐3.13) and subglottic pressure (kPa) (Left RLN: Coef = 0.01, CI = 0.011‐0.014; Right RLN: Coef = 0.01, CI = 0.009‐0.013) increased with increasing RLN stimulation. Subglottic airflow (l/min) (Left RLN: Coef = −0.29, CI = −0.36‐0.20; Right RLN: Coef = −0.29, CI = −0.37‐0.21) and fundamental frequency (Hz) (Left RLN: Coef = −0.59, CI = −0.76‐0.42; Right RLN: Coef = −0.24, CI = −0.41‐0.07) decreased with increasing RLN stimulation. Conclusion Higher levels of asymmetric laryngeal activation are associated with increasing phonatory instabilities. These results can be applied to further characterize asymmetric laryngeal states that lead to vocal irregulaties.