Vocal fold dynamics in a synthetic self-oscillating model: Contact pressure and dissipated-energy dose

Uncertainty of Spatial Segmentation of High-Speed Videoendoscopy and Its Temporal and Spatial Dependency

OBJECTIVE

Spatial segmentation of high-speed videoendoscopy (HSV) is the process that detects the edges of the vocal folds and represents them in analytic form. The level of spatial segmentation uncertainty (ie, how close vs. far apart different experts marked the edges of the vocal folds) can have a great impact on the level of uncertainty of the final measures (ie, their dispersion). This study quantified the uncertainty of spatial segmentation and investigated its dependency on the phase of the glottal cycle and the location of vocal fold edges along the anterior-posterior direction.

METHOD

Three experts manually segmented the vocal fold edges of twelve HSV recordings using an iterative process consisting of an initial segmentation followed by a blinded reconciliation phase. Segmentation uncertainty was computed as the distance in pixels between the three-segmented edges at the end of the iterative process. The relationships between segmentation uncertainty and different sections of the glottis along the anterior-posterior direction and the relationships between segmentation uncertainty and different phases of the glottal cycle were quantified.

RESULTS

Segmentation uncertainties of the anterior and the posterior sections of the glottis were significantly higher than the middle section, while uncertainty of the anterior section was the highest and 40% larger than the middle section. The average segmentation uncertainty and normalized glottal area were positively correlated. Segmentation uncertainty of the most open glottal configurations was 31% larger than the most closed glottal configuration.

CONCLUSION

The uncertainty of spatial segmentation of the vocal fold edges depends on the phase of the glottal cycle and the location of the edge along the anterior-posterior direction; hence, it is expected for different HSV measures to have different levels of uncertainties. The implications of these findings for vocal fold velocity measures are discussed. Additionally, the findings from this study could provide direction for future automated spatial segmentation methods and for creating a robust and reliable automated HSV processing pipeline.

Effects of Recording Condition and Number of Monitored Days on the Discriminative Power of the Daily Phonotrauma Index

PURPOSE

The Daily Phonotrauma Index (DPI) can quantify pathophysiological mechanisms associated with daily voice use in individuals with phonotraumatic vocal hyperfunction (PVH). Since DPI was developed based on weeklong ambulatory voice monitoring, this study investigated if DPI can achieve comparable performance using (a) short laboratory speech tasks and (b) fewer than 7 days of ambulatory data.

METHOD

An ambulatory voice monitoring system recorded the vocal function/behavior of 134 females with PVH and vocally healthy matched controls in two different conditions. In the laboratory, the participants read the first paragraph of the Rainbow Passage and produced spontaneous speech (in-lab data). They were then monitored for 7 days (in-field data). Separate DPI models were trained from the in-lab and in-field data using the standard deviation of the difference between the magnitude of the first two harmonics (H1-H2) and the skewness of neck-surface acceleration magnitude. First, 10-fold cross-validation evaluated the classification performance of the in-lab and in-field DPIs. Second, the effect of the number of ambulatory monitoring days on the accuracy of in-field DPI classification was quantified.

RESULTS

The average in-lab DPI accuracy computed from the Rainbow Passage and spontaneous speech were 57.9% and 48.9%, respectively, which are close to chance performance. The average classification accuracy of the in-field DPI was significantly higher with a very large effect size (73.4%, Cohen's d = 1.8). Next, the average in-field DPI accuracy increased from 66.5% for 1 day to 75.0% for 7 days, with the gain of including an additional day on accuracy dropping below 1 percentage point after 4 days.

CONCLUSIONS

The DPI requires ambulatory monitoring data as its discriminative power diminished significantly once computed from short in-lab recordings. Additionally, ambulatory monitoring should sample multiple days to achieve robust performance. The result of this research note can be used to make an informed decision about the trade-off between classification accuracy and cost of data collection.

Biophysical aspects of mechanotransduction in cells and their physiological/biological implications in vocal fold vibration: a narrative review

Mechanotransduction is a crucial property in all organisms, modulating cellular behaviors in response to external mechanical stimuli. Given the high mobility of vocal folds, it is hypothesized that mechanotransduction significantly contributes to their tissue homeostasis. Recent studies have identified mechanosensitive proteins in vocal fold epithelia, supporting this hypothesis. Voice therapy, which, involves the mobilization of vocal folds, aims to rehabilitate vocal function and restore homeostasis. However, establishing a direct causal link between specific mechanical stimuli and therapeutic benefits is challenging due to the variability in voice therapy techniques. This challenge is further compounded when investigating biological benefits in humans. Vocal fold tissue cannot be biopsied without significant impairment of the vibratory characteristics of the vocal folds. Conversely, studies using vocal fold mimetic bioreactors have demonstrated that mechanical stimulation of vocal fold fibroblasts can lead to highly heterogeneous responses, depending on the nature and parameters of the induced vibration. These responses can either aid or impede vocal fold vibration at the physiological level. Future research is needed to determine the specific mechanical parameters that are biologically beneficial for vocal fold function.

Vocal Fold Dissipated Power in Females with Hyperfunctional Voice Disorders

OBJECTIVE

Phonotrauma has been hypothesized to be associated with prolonged and/or accumulated biomechanical stress on vocal fold tissue. This hypothesis can be tested using ambulatory monitoring of vocal fold dissipated power, which requires a reliable method for its noninvasive estimation during the activity of daily living. The first aim of this study was to show that a laboratory-based estimate of vocal fold dissipated power computed from intraoral pressure (IOP) has significant discriminative power in individuals with phonotraumatic vocal hyperfunction (PVH). Considering that estimation of subglottal pressure from IOP is not practical for ambulatory applications, an alternative approach should be used. The second aim of this study was to test the impact of two alternative methods for the estimation of subglottal pressure on the discriminative power of vocal fold dissipated power in individuals with PVH and, hence, to provide an evidence-based recommendation for future ambulatory monitoring studies of vocal fold dissipated power.

METHOD

Four groups of adult females were included in this study: 16 individuals with PVH, 16 individuals with nonphonotraumatic vocal hyperfunction (NPVH), and two groups of vocally typical controls matched to the participants in each patient group in terms of age and occupation. Each participant produced strings of five consecutive /pae/ syllables while wearing a pneumotachograph mask with an IOP tube. Neck-surface accelerometer and acoustic signals were recorded simultaneously using an ambulatory voice monitor and a head-mounted microphone, respectively. IOP was used to estimate subglottal pressure and subject-specific calibration factors were determined for the estimation of subglottal pressure from the accelerometer signal.

RESULTS

(1) Individuals with PVH had significantly higher dissipated power than controls (P = 0.001, Cohen's D=1.31) when the intraoral estimate of subglottal pressure was used in the computation of dissipated power. (2) The difference between the dissipated power of individuals with NPVH and their matched controls was not significant. (3) When microphone-based sound pressure levels was used for the estimation of subglottal pressure, the difference between individuals with PVH and their matched controls vanished (P = 0.23). (4) When subject-specific estimation of subglottal pressure from the accelerometer was used, the discriminative power returned with a very large effect size (P = 0.001, D=1.38).

CONCLUSION

Increased dissipated power is sensitive and specific to individuals with PVH among individuals with hyperfunctional voice disorders. The results provide evidence that accelerometer-based estimate of energy dissipation dose (power integrated over time) during daily life could be clinically useful.

Synthetic, self-oscillating vocal fold models for voice production researcha)

Sound for the human voice is produced by vocal fold flow-induced vibration and involves a complex coupling between flow dynamics, tissue motion, and acoustics. Over the past three decades, synthetic, self-oscillating vocal fold models have played an increasingly important role in the study of these complex physical interactions. In particular, two types of models have been established: "membranous" vocal fold models, such as a water-filled latex tube, and "elastic solid" models, such as ultrasoft silicone formed into a vocal fold-like shape and in some cases with multiple layers of differing stiffness to mimic the human vocal fold tissue structure. In this review, the designs, capabilities, and limitations of these two types of models are presented. Considerations unique to the implementation of elastic solid models, including fabrication processes and materials, are discussed. Applications in which these models have been used to study the underlying mechanical principles that govern phonation are surveyed, and experimental techniques and configurations are reviewed. Finally, recommendations for continued development of these models for even more lifelike response and clinical relevance are summarized.

The effect of swelling on vocal fold kinematics and dynamics

Swelling in the vocal folds is caused by the local accumulation of fluid, and has been implicated as a phase in the development of phonotraumatic vocal hyperfunction and related structural pathologies, such as vocal fold nodules. It has been posited that small degrees of swelling may be protective, but large amounts may lead to a vicious cycle wherein the engorged folds lead to conditions that promote further swelling, leading to pathologies. As a first effort to explore the mechanics of vocal fold swelling and its potential role in the etiology of voice disorders, this study employs a finite-element model with swelling confined to the superficial lamina propria, which changes the volume, mass, and stiffness of the cover layer. The impacts of swelling on a number of vocal fold kinematic and damage measures, including von Mises stress, internal viscous dissipation, and collision pressure, are presented. Swelling has small but consistent effects on voice outputs, including a reduction in fundamental frequency with increasing swelling (10 Hz at 30 % swelling). Average von Mises stress decreases slightly for small degrees of swelling but increases at large magnitudes, consistent with expectations for a vicious cycle. Both viscous dissipation and collision pressure consistently increase with the magnitude of swelling. This first effort at modeling the impact of swelling on vocal fold kinematics, kinetics, and damage measures highlights the complexity with which phonotrauma can influence performance metrics. Further identification and exploration of salient candidate measures of damage and refined studies coupling swelling with local phonotrauma are expected to shed further light on the etiological pathways of phonotraumatic vocal hyperfunction.

Effect of Ligament Fibers on Dynamics of Synthetic, Self-Oscillating Vocal Folds in a Biomimetic Larynx Model

Synthetic silicone larynx models are essential for understanding the biomechanics of physiological and pathological vocal fold vibrations. The aim of this study is to investigate the effects of artificial ligament fibers on vocal fold vibrations in a synthetic larynx model, which is capable of replicating physiological laryngeal functions such as elongation, abduction, and adduction. A multi-layer silicone model with different mechanical properties for the musculus vocalis and the lamina propria consisting of ligament and mucosa was used. Ligament fibers of various diameters and break resistances were cast into the vocal folds and tested at different tension levels. An electromechanical setup was developed to mimic laryngeal physiology. The measurements included high-speed video recordings of vocal fold vibrations, subglottal pressure and acoustic. For the evaluation of the vibration characteristics, all measured values were evaluated and compared with parameters from ex and in vivo studies. The fundamental frequency of the synthetic larynx model was found to be approximately 200-520 Hz depending on integrated fiber types and tension levels. This range of the fundamental frequency corresponds to the reproduction of a female normal and singing voice range. The investigated voice parameters from vocal fold vibration, acoustics, and subglottal pressure were within normal value ranges from ex and in vivo studies. The integration of ligament fibers leads to an increase in the fundamental frequency with increasing airflow, while the tensioning of the ligament fibers remains constant. In addition, a tension increase in the fibers also generates a rise in the fundamental frequency delivering the physiological expectation of the dynamic behavior of vocal folds.

Examining the influence of epithelium layer modeling approaches on vocal fold kinematics and kinetics

Grouping the thin epithelium and thicker superficial lamina propria layers into a single cover layer has been widely adopted in finite element vocal fold models. Recent silicone vocal fold studies have suggested, however, that inclusion of a distinct epithelial layer leads to more physiologically representative motion. This study systematically explores the ramifications of incorporating an epithelial layer into a cover grouping for finite element vocal fold modeling. A membrane model for the epithelium is introduced to facilitate parametric investigation by reducing the mesh density requirement of the epithelium into a single infinitesimally thin layer. Excluding the epithelium entirely leads to increased energy in higher order modes and larger inferior-superior excursion of the folds. Integrating the epithelium into a cover layer with volume-weighted average stiffness results in similar kinematics to that of a model treating the epithelium as a distinct layer. However, the internal stress/strain and contact pressure during collision are higher, and viscous dissipation is lower, when the epithelium is integrated into the cover. Thus, careful treatment of the epithelium is recommended for finite element studies, particularly when employing the models for estimating measures dependent upon internal stress/strain and/or collision pressure, such as vocal dose.

Effect of nodule size and stiffness on phonation threshold and collision pressures in a synthetic hemilaryngeal vocal fold model

Synthetic vocal fold (VF) replicas were used to explore the role of nodule size and stiffness on kinematic, aerodynamic, and acoustic measures of voiced speech production. Emphasis was placed on determining how changes in collision pressure may contribute to the development of phonotrauma. This was performed by adding spherical beads with different sizes and moduli of elasticity at the middle of the medial surface of synthetic silicone VF models, representing nodules of varying size and stiffness. The VF models were incorporated into a hemilaryngeal flow facility. For each case, self-sustained oscillations were investigated at the phonation threshold pressure. It was found that increasing the nodule diameter increased the open quotient, phonation threshold pressure, and phonation threshold flow rate. However, these values did not change considerably as a function of the modulus of elasticity of the nodule. Nevertheless, the ratio of collision pressure to subglottal pressure increased significantly for both increasing nodule size and stiffness. This suggests that over time, both growth in size and fibrosis of nodules will lead to an increasing cycle of compensatory vocal hyperfunction that accelerates phonotrauma.

Collision Pressure and Dissipated Power Dose in a Self-Oscillating Silicone Vocal Fold Model With a Posterior Glottal Opening

PURPOSE

The goal of this study was to experimentally evaluate how compensating for the adverse acoustic effects of a posterior glottal opening (PGO) by increasing subglottal pressure and changing supraglottal compression, as have been associated with vocal hyperfunction, influences the risk of vocal fold (VF) trauma.

METHOD

A self-oscillating synthetic silicone model of the VFs with an airflow bypass that modeled a PGO was investigated in a hemilaryngeal flow facility. The influence of compensatory mechanisms on collision pressure and dissipated collision power was investigated for different PGO areas and supraglottal compression. Compensatory behaviors were mimicked by increasing the subglottal pressure to achieve a target sound pressure level (SPL).

RESULTS

Increasing the subglottal pressure to compensate for decreased SPL due to a PGO produced higher values for both collision pressure and dissipated collision power. Whereas a 10-mm² PGO area produced a 12% increase in the peak collision pressure, the dissipated collision power increased by 122%, mainly due to an increase in the magnitude of the collision velocity. This suggests that the value of peak collision pressure may not fully capture the mechanisms by which phonotrauma occurs. It was also found that an optimal value of supraglottal compression exists that maximizes the radiated SPL, indicating the potential utility of supraglottal compression as a compensatory mechanism.

CONCLUSIONS

Larger PGO areas are expected to increase the risk of phonotrauma due to the concomitant increase in dissipated collision power associated with maintaining SPL. Furthermore, the risk of VF damage may not be fully characterized by only the peak collision pressure.

Optimization of Synthetic Vocal Fold Models for Glottal Closure

Synthetic, self-oscillating models of the human vocal folds are used to study the complex and inter-related flow, structure, and acoustical aspects of voice production. The vocal folds typically collide during each cycle, thereby creating a brief period of glottal closure that has important implications for flow, acoustic, and motion-related outcomes. Many previous synthetic models, however, have been limited by incomplete glottal closure during vibration. In this study, a low-fidelity, two-dimensional, multilayer finite element model of vocal fold flow-induced vibration was coupled with a custom genetic algorithm optimization code to determine geometric and material characteristics that would be expected to yield physiologically-realistic frequency and closed quotient values. The optimization process yielded computational models that vibrated with favorable frequency and closed quotient characteristics. A tradeoff was observed between frequency and closed quotient. A synthetic, self-oscillating vocal fold model with geometric and material properties informed by the simulation outcomes was fabricated and tested for onset pressure, oscillation frequency, and closed quotient. The synthetic model successfully vibrated at a realistic frequency and exhibited a nonzero closed quotient. The methodology described in this study provides potential direction for fabricating synthetic models using isotropic silicone materials that can be designed to vibrate with physiologically-realistic frequencies and closed quotient values. The results also show the potential for a low-fidelity model optimization approach to be used to tune synthetic vocal fold model characteristics for specific vibratory outcomes.

Direct measurement and modeling of intraglottal, subglottal, and vocal fold collision pressures during phonation in an individual with a hemilaryngectomy

The purpose of this paper is to report on the first in vivo application of a recently developed transoral, dual-sensor pressure probe that directly measures intraglottal, subglottal, and vocal fold collision pressures during phonation. Synchronous measurement of intraglottal and subglottal pressures was accomplished using two miniature pressure sensors mounted on the end of the probe and inserted transorally in a 78-year-old male who had previously undergone surgical removal of his right vocal fold for treatment of laryngeal cancer. The endoscopist used one hand to position the custom probe against the surgically medialized scar band that replaced the right vocal fold and used the other hand to position a transoral endoscope to record laryngeal high-speed videoendoscopy of the vibrating left vocal fold contacting the pressure probe. Visualization of the larynx during sustained phonation allowed the endoscopist to place the dual-sensor pressure probe such that the proximal sensor was positioned intraglottally and the distal sensor subglottally. The proximal pressure sensor was verified to be in the strike zone of vocal fold collision during phonation when the intraglottal pressure signal exhibited three characteristics: an impulsive peak at the start of the closed phase, rounded peak during the open phase, and minimum value around zero immediately preceding the impulsive peak of the subsequent phonatory cycle. Numerical voice production modeling was applied to validate model-based predictions of vocal fold collision pressure using kinematic vocal fold measures. The results successfully demonstrated feasibility of in vivo measurement of vocal fold collision pressure in an individual with a hemilaryngectomy, motivating ongoing data collection that is designed to aid in the development of vocal dose measures that incorporate vocal fold impact collision and stresses.

Vocal fold dynamics in a synthetic self-oscillating model: Intraglottal aerodynamic pressure and energy

Self-sustained oscillations of the vocal folds (VFs) during phonation are the result of the energy exchange between the airflow and VF tissue. Understanding this mechanism requires accurate investigation of the aerodynamic pressures acting on the VF surface during oscillation. A self-oscillating silicone VF model was used in a hemilaryngeal flow facility to measure the time-varying pressure distribution along the inferior-superior thickness of the VF and at four discrete locations in the anterior-posterior direction. It was found that the intraglottal pressures during the opening and closing phases of the glottis are highly dependent on three-dimensional and unsteady flow behaviors. The measured aerodynamic pressures and estimates of the medial surface velocity were used to compute the intraglottal energy transfer from the airflow to the VFs. The energy was greatest at the anterior-posterior midline and decreased significantly toward the anterior/posterior endpoints. The findings provide insight into the dynamics of the VF oscillation and potential causes of some VF disorders.

Vocal fold dynamics in a synthetic self-oscillating model: Contact pressure and dissipated-energy dose

The energy dissipated during vocal fold (VF) contact is a predictor of phonotrauma. Difficulty measuring contact pressure has forced prior energy dissipation estimates to rely upon generalized approximations of the contact dynamics. To address this shortcoming, contact pressure was measured in a self-oscillating synthetic VF model with high spatiotemporal resolution using a hemilaryngeal configuration. The approach yields a temporal resolution of less than 0.26 ms and a spatial resolution of 0.254 mm in the inferior-superior direction. The average contact pressure was found to be 32% of the peak contact pressure, 60% higher than the ratio estimated in prior studies. It was found that 52% of the total power was dissipated due to collision. The power dissipated during contact was an order of magnitude higher than the power dissipated due to internal friction during the non-contact phase of oscillation. Both the contact pressure magnitude and dissipated power were found to be maximums at the mid anterior-posterior position, supporting the idea that collision is responsible for the formation of benign lesions, which normally appear at the middle third of the VF.