Vocal instabilities in a three-dimensional body-cover phonation model

A computational study of the influence of thyroarytenoid and cricothyroid muscle interaction on vocal fold dynamics in an MRI-based human laryngeal model

A human laryngeal model, incorporating all the cartilages and the intrinsic muscles, was reconstructed based on MRI data. The vocal fold was represented as a multilayer structure with detailed inner components. The activation levels of the thyroarytenoid (TA) and cricothyroid (CT) muscles were systematically varied from zero to full activation allowing for the analysis of their interaction and influence on vocal fold dynamics and glottal flow. The finite element method was employed to calculate the vocal fold dynamics, while the one-dimensional Bernoulli equation was utilized to calculate the glottal flow. The analysis was focused on the muscle influence on the fundamental frequency (fo). We found that while CT and TA  activation increased the fo in most of the conditions, TA activation resulted in a frequency drop when it was moderately activated. We show that this frequency drop was associated with the sudden increase of the vertical motion when the vibration transited from involving the whole tissue to mainly in the cover layer. The transition of the vibration pattern was caused by the increased body-cover stiffness ratio that resulted from TA activation.

Contribution of Undesired Medial Surface Shape to Suboptimal Voice Outcome After Medialization Laryngoplasty

OBJECTIVES

Voice production in pathological conditions or after surgical intervention often involves undesired medial surface shape such as reduced vertical thickness and/or left-right asymmetry in medial surface shape. The effect of such undesired medial surface on voice production remains unclear, and is often not taken into consideration during planning of surgical intervention, due to difficulty of imaging the medial surface in patients. This study aims to better understand how voice outcomes are impacted by undesired medial surface shape.

METHODS

Computational simulations were conducted to parametrically manipulate medial surface shape and stiffness and observe its consequence on voice production.

RESULTS

The results showed that undesired medial surface shape can result in incomplete glottal closure, weak voice production, increased phonation threshold, and significantly reduced vocal efficiency, particularly in the presence of left-right stiffness asymmetry.

CONCLUSIONS

In addition to approximating the vocal folds, medialization laryngoplasty should aim to sufficiently increase medial surface thickness, which may improve voice outcomes in patients whose voices remain unsatisfactory or suboptimal after initial intervention. While a divergent implant may increase medial surface thickness, precise implant placement in anticipation of tissue and implant deformation during the insertion process is equally important in order to achieve desired medial surface shape and optimal voice outcomes.

Asymmetric triangular body-cover model of the vocal folds with bilateral intrinsic muscle activation

Many voice disorders are linked to imbalanced muscle activity and known to exhibit asymmetric vocal fold vibration. However, the relation between imbalanced muscle activation and asymmetric vocal fold vibration is not well understood. This study introduces an asymmetric triangular body-cover model of the vocal folds, controlled by the activation of bilateral intrinsic laryngeal muscles, to investigate the effects of muscle imbalance on vocal fold oscillation. Various scenarios were considered, encompassing imbalance in individual muscles and muscle pairs, as well as accounting for asymmetry in lumped element parameters. Measurements of amplitude and phase asymmetries were employed to match the oscillatory behavior of two pathological cases: unilateral paralysis and muscle tension dysphonia. The resulting simulations exhibit muscle imbalance consistent with expectations in the composition of these voice disorders, yielding asymmetries exceeding 30% for paralysis and below 5% for dysphonia. This underscores the relevance of muscle imbalance in representing phonatory scenarios and its potential for characterizing asymmetry in vocal fold vibration.

Asymmetric triangular body-cover model of the VFs with bilateral intrinsic muscle activation

Many voice disorders are linked to imbalanced muscle activity and known to exhibit asymmetric vocal fold vibration. However, the relation between imbalanced muscle activation and asymmetric vocal fold vibration is not well understood. This study introduces an asymmetric triangular body-cover model of the vocal folds, controlled by the activation of intrinsic laryngeal muscles, to investigate the effects of muscle imbalance on vocal fold oscillation. Various scenarios were considered, encompassing imbalance in individual muscles and muscle pairs, as well as accounting for asymmetry in lumped element parameters. The results highlight the antagonistic effect between the thyroarytenoid and cricothyroid muscles on the elastic and mass components of the vocal folds, as well as the impact on the vocal process from the imbalance in the lateral cricoarytenoid and interarytenoid adductor muscles. Measurements of amplitude and phase asymmetry were employed to emulate the oscillatory behavior of two pathological cases: unilateral paralysis and muscle tension dysphonia. The resulting simulations exhibit muscle imbalance consistent with expectations in the composition of these voice disorders, yielding asymmetries exceeding 30% for paralysis and below 5% for dysphonia. This underscores the versatility of muscle imbalance in representing phonatory scenarios and its potential for characterizing asymmetry in vocal fold vibration.

The influence of sensor size on experimental measurement accuracy of vocal fold contact pressure

The vocal folds experience repeated collision during phonation. The resulting contact pressure is often considered to play an important role in vocal fold injury, and has been the focus of many experimental studies. In this study, vocal fold contact pattern and contact pressure during phonation were numerically investigated. The results show that vocal fold contact in general occurs within a horizontal strip on the medial surface, first appearing at the inferior medial surface and propagating upward. Because of the localized and travelling nature of vocal fold contact, sensors of a finite size may significantly underestimate the peak vocal fold contact pressure, particularly for vocal folds of low transverse stiffness. This underestimation also makes it difficult to identify the contact pressure peak in the intraglottal pressure waveform. These results showed that the vocal fold contact pressure reported in previous experimental studies may have significantly underestimated the actual values. It is recommended that contact pressure sensors with a diameter no greater than 0.4 mm are used in future experiments to ensure adequate accuracy in measuring the peak vocal fold contact pressure during phonation.

The influence of source-filter interaction on the voice source in a three-dimensional computational model of voice production

The goal of this computational study is to quantify global effects of vocal tract constriction at various locations (false vocal folds, aryepiglottic folds, pharynx, oral cavity, and lips) on the voice source across a large range of vocal fold conditions. The results showed that while inclusion of a uniform vocal tract had notable effects on the voice source, further constricting the vocal tract only had small effects except for conditions of extreme constriction, at which constrictions at any location along the vocal tract decreased the mean and peak-to-peak amplitude of the glottal flow waveform. Although narrowing in the epilarynx increased the normalized maximum flow declination rate, vocal tract constriction in general slightly reduced the source strength and high-frequency harmonic production at the glottis, except for a limited set of vocal fold conditions (e.g., soft, long vocal folds subject to relatively high pressure). This suggests that simultaneous laryngeal and vocal tract adjustments are required to maximize source-filter interaction. While vocal tract adjustments are often assumed to improve voice production, our results indicate that such improvements are mainly due to changes in vocal tract acoustic response rather than improved voice production at the glottis.

Voice Feature Selection to Improve Performance of Machine Learning Models for Voice Production Inversion

OBJECTIVE

Estimation of physiological control parameters of the vocal system from the produced voice outcome has important applications in clinical management of voice disorders . Previously we developed a simulation-based neural network for estimation of vocal fold geometry, mechanical properties, and subglottal pressure from voice outcome features that characterize the acoustics of the produced voice. The goals of this study are to (1) explore the possibility of improving the estimation accuracy of physiological control parameters by including voice outcome features characterizing vocal fold vibration; and (2) identify voice feature sets that optimize both estimation accuracy and robustness to measurement noise.

METHODS

Feedforward neural networks are trained to solve the inversion problem of estimating the physiological control parameters of a three-dimensional body-cover vocal fold model from different sets of voice outcome features that characterize the simulated voice acoustics, glottal flow, and vocal fold vibration. A sensitivity analysis is then performed to evaluate the contribution of individual voice features to the overall performance of the neural networks in estimating the physiologic control parameters.

RESULTS AND CONCLUSIONS

While including voice outcome features characterizing vocal fold vibration increases estimation accuracy, it also reduces the network's robustness to measurement noise, due to high sensitivity of network performance to voice outcome features measuring the absolute amplitudes of the glottal flow and area waveforms, which are also difficult to measure accurately in practical applications. By excluding such glottal flow-based features and replacing glottal area-based features by their normalized counterparts, we are able to significantly improve both estimation accuracy and robustness to noise. We further show that similar estimation accuracy and robustness can be achieved with an even smaller set of voice outcome features by excluding features of small sensitivity.

Impact of the Paraglottic Space on Voice Production in an MRI-Based Vocal Fold Model

OBJECTIVE

While the vocal fold is in direct contact anteriorly with the thyroid cartilage, posteriorly the vocal fold connects to the thyroid cartilage through a soft tissue layer in the paraglottic space. Currently the paraglottic space is often neglected in computational models of phonation, in which a fixed boundary condition is often imposed on the lateral surface of the vocal fold. The goal of this study was to investigate the effect of the paraglottic space on voice production in an MRI-based vocal fold model, and how this effect may be counteracted by vocal fold stiffening due to laryngeal muscle activation.

METHODS

Parametric simulation study using an MRI-based computational vocal fold model.

RESULTS

The results showed that the presence of the paraglottic space increased the mean and amplitude of the glottal area waveform, decreased the phonation frequency and closed quotient. For the particular vocal fold geometry used in this study, the presence of the paraglottic space also reduced the occurrence of irregular vocal fold vibration. These effects of the paraglottic space became smaller with increasing paraglottic space stiffness and to a lesser degree with vocal fold stiffening.

CONCLUSIONS

The results suggest that the paraglottic space may be neglected in qualitative evaluations of normal phonation, but needs to be included in simulations of pathological phonation or vocal fold posturing.

Vocal Fold Vertical Thickness in Human Voice Production and Control: A Review

While current voice research often focuses on laryngeal adjustments in a two-dimensional plane from a superior endoscopic view, recent computational simulations showed that vocal control is three-dimensional and the medial surface vertical thickness plays an important role in regulating the glottal closure pattern and the spectral shape of the produced voice. In contrast, while a small glottal gap is required to initiate and sustain phonation, further changes in the glottal gap within this small range have only small effects on glottal closure and spectral shape. Vocal fold stiffness, particularly along the anterior-posterior direction, plays an important role in pitch control but has only a small effect on glottal closure and spectral shape. These results suggest that voice research should pay more attention to medial surface shape in the vertical dimension. Future studies in a large population of both normal speakers and patients are needed to better characterize the three-dimensional medial surface shape, its variability between speakers, changes throughout the life span, and how it is impacted by voice disorders and clinical interventions. The implications for voice pedagogy and clinical intervention are discussed.

Oral vibratory sensations during voice production at different laryngeal and semi-occluded vocal tract configurations

Voice therapy often emphasizes vibratory sensations in the front part of the vocal tract during phonation to improve vocal efficiency. It remains unclear what laryngeal and vocal tract adjustments are elicited in speakers by this emphasis on oral vibratory sensations. Using a three-dimensional phonation model, this study aims to identify laryngeal and epilaryngeal adjustments that might produce maximal oral vibratory sensations during phonation, as quantified by the oral sound pressure level (SPL), and thus are likely to be elicited in voice therapy at different semi-occluded vocal tract configurations. Results show that maximum oral SPL occurs at intermediate vocal fold adduction configurations characterized by a trade-off between glottal gap and vocal fold vertical thickness. Epilaryngeal tube narrowing further increases the oral SPL in an open vocal tract, but has little effect on oral SPL in semi-occluded vocal tracts. Laryngeal and epilaryngeal configurations producing the maximum oral SPL generally have lower peak vocal fold contact pressure when producing a target output SPL. These favorable configurations are more easily identified in open vocal tracts than semi-occluded vocal tracts. However, semi-occlusion increases both the mean and dynamic oral pressure, which may familiarize speakers with oral vibratory sensations and facilitate adoption of favorable laryngeal configurations.

Analysis of vibratory mode changes in symmetric and asymmetric activation of the canine larynx

Investigations of neuromuscular control of voice production have primarily focused on the roles of muscle activation levels, posture, and stiffness at phonation onset. However, little work has been done investigating the stability of the phonation process in regards to spontaneous changes in vibratory mode of vocal fold oscillation as a function of neuromuscular activation. We evaluated 320 phonatory conditions representing combinations of superior and recurrent laryngeal nerve (SLN and RLN) activations in an in vivo canine model of phonation. At each combination of neuromuscular input, airflow was increased linearly to reach phonation onset and beyond from 300 to 1400 mL/s. High-speed video and acoustic data were recorded during phonation, and spectrograms and glottal-area-based parameters were calculated. Vibratory mode changes were detected based on sudden increases or drops of local fundamental frequency. Mode changes occurred only when SLNs were concurrently stimulated and were more frequent for higher, less asymmetric RLN stimulation. A slight increase in amplitude and cycle length perturbation usually preceded the changes in the vibratory mode. However, no inherent differences between signals with mode changes and signals without were found.

Subject-Specific Computational Fluid-Structure Interaction Modeling of Rabbit Vocal Fold Vibration

A full three-dimensional (3D) fluid-structure interaction (FSI) study of subject-specific vocal fold vibration is carried out based on the previously reconstructed vocal fold models of rabbit larynges. Our primary focuses are the vibration characteristics of the vocal fold, the unsteady 3D flow field, and comparison with a recently developed 1D glottal flow model that incorporates machine learning. The 3D FSI model applies strong coupling between the finite-element model for the vocal fold tissue and the incompressible Navier-Stokes equation for the flow. Five different samples of the rabbit larynx, reconstructed from the magnetic resonance imaging (MRI) scans after the in vivo phonation experiments, are used in the FSI simulation. These samples have distinct geometries and a different inlet pressure measured in the experiment. Furthermore, the material properties of the vocal fold tissue were determined previously for each individual sample. The results demonstrate that the vibration and the intraglottal pressure from the 3D flow simulation agree well with those from the 1D flow model based simulation. Further 3D analyses show that the inferior and supraglottal geometries play significant roles in the FSI process. Similarity of the flow pattern with the human vocal fold is discussed. This study supports the effective usage of rabbit larynges to understand human phonation and will help guide our future computational studies that address vocal fold disorders.

Computational simulations of respiratory-laryngeal interactions and their effects on lung volume termination during phonation: Considerations for hyperfunctional voice disorders

Glottal resistance plays an important role in airflow conservation, especially in the context of high vocal demands. However, it remains unclear if laryngeal strategies most effective in controlling airflow during phonation are consistent with clinical manifestations of vocal hyperfunction. This study used a previously validated three-dimensional computational model of the vocal folds coupled with a respiratory model to investigate which laryngeal strategies were the best predictors of lung volume termination (LVT) and how these strategies' effects were modulated by respiratory parameters. Results indicated that the initial glottal angle and vertical thickness of the vocal folds were the best predictors of LVT regardless of subglottal pressure, lung volume initiation, and breath group duration. The effect of vertical thickness on LVT increased with the subglottal pressure-highlighting the importance of monitoring loudness during voice therapy to avoid laryngeal compensation-and decreased with increasing vocal fold stiffness. A positive initial glottal angle required an increase in vertical thickness to complete a target utterance, especially when the respiratory system was taxed. Overall, findings support the hypothesis that laryngeal strategies consistent with hyperfunctional voice disorders are effective in increasing LVT, and that conservation of airflow and respiratory effort may represent underlying mechanisms in those disorders.

Laryngeal strategies to minimize vocal fold contact pressure and their effect on voice production

The goal of this study is to identify laryngeal strategies that minimize vocal fold contact pressure while producing a target sound pressure level (SPL) using a three-dimensional voice production model. The results show that while the subglottal pressure and transverse stiffness can be manipulated to reduce the peak contact pressure, such manipulations also reduce the SPL, and are thus less effective in reducing contact pressure in voice tasks targeting a specific SPL level. In contrast, changes in the initial glottal angle and vocal fold vertical thickness that reduce the contact pressure also increase the SPL. Thus, in voice tasks targeting a specific SPL, such changes in the initial glottal angle and vertical thickness also lower the subglottal pressure, which further reduces the peak contact pressure. Overall the results show that for voice tasks with a target SPL level, vocal fold contact pressure can be significantly reduced by adopting a barely abducted glottal configuration or reducing the vocal fold vertical thickness. Aerodynamic measures are effective in identifying voice production with large initial glottal angles, but by themselves alone are not useful in differentiating hyperadducted vocal folds from barely abducted vocal folds, which may be better differentiated by closed quotient and voice type measures.

Estimation of vocal fold physiology from voice acoustics using machine learning

The goal of this study is to estimate vocal fold geometry, stiffness, position, and subglottal pressure from voice acoustics, toward clinical and other voice technology applications. Unlike previous voice inversion research that often uses lumped-element models of phonation, this study explores the feasibility of voice inversion using data generated from a three-dimensional voice production model. Neural networks are trained to estimate vocal fold properties and subglottal pressure from voice features extracted from the simulation data. Results show reasonably good estimation accuracy, particularly for vocal fold properties with a consistent global effect on voice production, and reasonable agreement with excised human larynx experiment.

Three-dimensional vocal fold structural change due to implant insertion in medialization laryngoplasty

Glottal insufficiency due to vocal fold paralysis, paresis, or atrophy often leads to degraded voice quality. One of the primary surgical intervention procedures to treat glottal insufficiency is medialization laryngoplasty, in which an implant is inserted through a lateral window on the thyroid cartilage to medialize the vocal folds. While the goal of medialization is to modify the vocal fold structure to restore normal phonation, few studies have attempted to quantify such structural changes of the vocal folds. The goal of this study is to quantify the three-dimensional structural changes of the vocal folds due to implant insertion in medialization laryngoplasty, and evaluate its potential effect on voice production. Medialization laryngoplasty were performed in excised human larynges using implants of different stiffness. Magnetic resonance images of the larynges were obtained with and without implant insertion. The results showed that implant insertion significantly changed the original body-cover structure of the vocal folds, with the implant taking over the large space used to be occupied by the original body layer and the vocal fold being stretched into a thin layer wrapped around the implant. The medial-lateral dimension of the vocal fold was significantly reduced from about 4 mm to 1 mm, and the vocal fold was stretched in the coronal plane by about 70%. It is hypothesized that use of implants with stiffness comparable to that of the vocal folds is beneficial because the degree of medialization can be adjusted without much negative effects on phonation frequency, phonation threshold pressure, or vibration amplitude.

Voice production in a MRI-based subject-specific vocal fold model with parametrically controlled medial surface shape

The goal of this study was to investigate how realistic changes in medial surface shape, as occur in human phonation, affect voice production. In a parametric magnetic resonance imaging-based three-dimensional vocal fold model, the superior and inferior portions of the medial surface were systematically manipulated to produce different medial surface contours similar to those observed in previous excised larynx and in vivo canine larynx experiments. Voice simulations were performed to investigate the differences in the resulting voice production. The results showed that both superior-medial bulging and inferior-medial bulging of the medial surface, which led to an increased vertical thickness and a more rectangular glottal configuration, increased the closed quotient of vocal fold vibration. Changes in medial surface shape also had significant effects on the phonation threshold pressure. The degree of these effects of changes in medial surface shape was larynx specific, and varied significantly depending on the vocal fold cross-sectional geometry and its variation along the anterior-posterior direction. The results suggest that, in addition to vocal fold approximation, surgical interventions of voice disorders should also aim at restoring a rectangular and sufficiently thick medial surface.

Vocal fold contact pressure in a three-dimensional body-cover phonation model

The goal of this study is to identify vocal fold geometric and mechanical conditions that are likely to produce large contact pressure and thus high risk of vocal fold injury. Using a three-dimensional computational model of phonation, parametric simulations are performed with co-variations in vocal fold geometry and stiffness, with and without a vocal tract. For each simulation, the peak contact pressure is calculated. The results show that the subglottal pressure and the transverse stiffness of the vocal folds in the coronal plane have the largest and most consistent effect on the peak contact pressure, indicating the importance of maintaining a balance between the subglottal pressure and transverse stiffness to avoiding vocal fold injury. The presence of a vocal tract generally increases the peak contact pressure, particularly for an open-mouth vocal tract configuration. While a low degree of vocal fold approximation significantly reduces vocal fold contact pressure, for conditions of moderate and tight vocal fold approximation changes in vocal fold approximation may increase or decrease the peak contact pressure. The effects of the medial surface thickness and vocal fold stiffness along the anterior-posterior direction are similarly inconsistent and vary depending on other control parameters and the vocal tract configuration.

Structural constitutive modeling of the anisotropic mechanical properties of human vocal fold lamina propria

The anisotropic mechanical properties of the vocal fold lamina propria play an important role in voice production and control. The goal of this study is to develop a constitutive model capable of predicting lamina propria elastic moduli along both the longitudinal and transverse directions under different conditions of vocal fold elongation, which can be used as input to reduced-order phonation models based on linear elasticity. A structurally-based constitutive model that links microstructural characteristics of the lamina propria to its macromechanical properties is proposed. The model prediction has been shown to agree reasonably well with recent biaxial tensile testing results.