Vestibular vocal fold behavior during phonation in unilateral vocal fold paralysis

Voice quality in patients with vocal fold paralysis can be affected by several factors, such as the position of the paralyzed vocal fold, its degree of atrophy, the configuration of its free edge, and the level differences between both vocal folds. Depending on the related vocal deficiency the patient will attempt to compensate using different maneuvers, such as increment of vocal tract and neck muscle contraction to improve glottal closure. This is probably one of the reasons why ventricular folds are frequently requested. The objective of this study is to analyze the behavior of the homolateral and contralateral vestibular folds to delineate patterns of vestibular motion during sustained phonation, in cases of unilateral vocal fold paralysis.     

Simulation of vocal fold impact pressures with a self-oscillating finite-element model

Vocal fold impact pressures were studied using a self-oscillating finite-element model capable of simulating vocal fold vibration and airflow. The calculated airflow pressure is applied on the vocal fold as the driving force. The airflow region is then adjusted according to the calculated vocal fold displacement. The interaction between airflow and the vocal folds produces a self-oscillating solution. Lung pressures between 0.2 and 2.5 kPa were used to drive this self-oscillating model. The spatial distribution of the impact pressure was studied. Studies revealed that the tissue collision during phonation produces a very large impact pressure which correlates with the lung pressure and glottal width. Larger lung pressure and a narrower glottal width increase the impact pressure. The impact pressure was found to be roughly the square root of lung pressure. In the inferior-superior direction, the maximum impact pressure is related to the narrowest glottis. In the anterior-posteriorfirection, the greatest impact pressure appears at the midpoint of the vocal fold. The match between our numerical simulations and clinical observations suggests that this self-oscillating finite-element model might be valuable for predicting mechanical trauma of the vocal folds.     

Effect of inferior surface angle on the self-oscillation of a computational vocal fold model

Geometry of the human vocal folds strongly influences their oscillatory motion. While the effect of intraglottal geometry on phonation has been widely investigated, the study of the geometry of the inferior surface of the vocal folds has been limited. In this study the way in which the inferior vocal fold surface angle affects vocal fold vibration was explored using a two-dimensional, self-oscillating finite element vocal fold model. The geometry was parameterized to create models with five different inferior surface angles. Four of the five models exhibited self-sustained oscillations. Comparisons of model motion showed increased vertical displacement and decreased glottal width amplitude with decreasing inferior surface angle. In addition, glottal width and air flow rate waveforms changed as the inferior surface angle was varied. Structural, rather than aerodynamic, effects are shown to be the cause of the changes in model response as the inferior surface angle was varied. Supporting data including glottal pressure distribution, average intraglottal pressure, energy transfer, and flow separation point locations are discussed, and suggestions for future research are given.     

Pressure-flow relationships during phonation in the canine larynx

Measurements of air pressure and flow were made using an in vivo canine model of the larynx. Subglottic pressures at varying flow rates were taken during phonation induced by laryngeal nerve stimulation. Results showed that during constant vocal fold stiffness, subglottic pressure rose slightly with increased air flow. The larynx in the in vivo canine model exhibited a flow-dependent decrease in laryngeal airway resistance. Increasing flow rate was associated with an increase in frequency of phonation and open quotient, as measured glottographically. Results from this experiment were compared with a theoretical two-mass model of the larynx and other theoretical models of phonation. The influence of aerodynamic forces on glottal vibration is explained by increased lateral excursion of the vocal folds during the open interval and shortening of the closed interval during the glottal cycle.     

Influence of supraglottal structures on the glottal jet exiting a two-layer synthetic, self-oscillating vocal fold model

A synthetic two-layer, self-oscillating, life-size vocal fold model was used to study the influence of the vocal tract and false folds on the glottal jet. The model vibrated at frequencies, pressures, flow rates, and amplitudes consistent with human phonation, although some differences in behavior between the model and the human vocal folds are noted. High-speed images of model motion and flow visualization were acquired. Phase-locked ensemble-averaged glottal jet velocity measurements using particle image velocimetry (PIV) were acquired with and without an idealized vocal tract, with and without false folds. PIV data were obtained with varying degrees of lateral asymmetric model positioning. Glottal jet velocity magnitudes were consistent with those measured using excised larynges. A starting vortex was observed in all test cases. The false folds interfered with the starting vortex, and in some cases vortex shedding from the false folds was observed. In asymmetric cases without false folds, the glottal jet tended to skew toward the nearest wall; with the false folds, the opposite trend was observed. rms velocity calculations showed the jet shear layer and laminar core. The rms velocities were higher in the vocal tract cases compared to the open jet and false fold cases.     

Phonation threshold pressure and onset frequency in a two-layer physical model of the vocal folds

The influence of vocal fold geometry and stiffness on phonation onset was experimentally investigated using a body-cover physical model of the vocal folds. Results showed that a lower phonation threshold pressure and phonation onset frequency can be achieved by reducing body-layer or cover-layer stiffness, reducing medial surface thickness, or increasing cover-layer depth. Increasing body-layer stiffness also restricted vocal fold motion to the cover layer and reduced prephonatory glottal opening. Excitation of anterior-posterior modes was also observed, particularly for large values of the body-cover stiffness ratio. The results of this study were also discussed in relation to previous theoretical and experimental studies.     

Three-dimensional biomechanical properties of human vocal folds: parameter optimization of a numerical model to match in vitro dynamics

The human voice signal originates from the vibrations of the two vocal folds within the larynx. The interactions of several intrinsic laryngeal muscles adduct and shape the vocal folds to facilitate vibration in response to airflow. Three-dimensional vocal fold dynamics are extracted from in vitro hemilarynx experiments and fitted by a numerical three-dimensional-multi-mass-model (3DM) using an optimization procedure. In this work, the 3DM dynamics are optimized over 24 experimental data sets to estimate biomechanical vocal fold properties during phonation. Accuracy of the optimization is verified by low normalized error (0.13 ± 0.02), high correlation (83% ± 2%), and reproducible subglottal pressure values. The optimized, 3DM parameters yielded biomechanical variations in tissue properties along the vocal fold surface, including variations in both the local mass and stiffness of vocal folds. That is, both mass and stiffness increased along the superior-to-inferior direction. These variations were statistically analyzed under different experimental conditions (e.g., an increase in tension as a function of vocal fold elongation and an increase in stiffness and a decrease in mass as a function of glottal airflow). The study showed that physiologically relevant vocal fold tissue properties, which cannot be directly measured during in vivo human phonation, can be captured using this 3D-modeling technique.     

Investigating acoustic correlates of human vocal fold vibratory phase asymmetry through modeling and laryngeal high-speed videoendoscopy

Vocal fold vibratory asymmetry is often associated with inefficient sound production through its impact on source spectral tilt. This association is investigated in both a computational voice production model and a group of 47 human subjects. The model provides indirect control over the degree of left-right phase asymmetry within a nonlinear source-filter framework, and high-speed videoendoscopy provides in vivo measures of vocal fold vibratory asymmetry. Source spectral tilt measures are estimated from the inverse-filtered spectrum of the simulated and recorded radiated acoustic pressure. As expected, model simulations indicate that increasing left-right phase asymmetry induces steeper spectral tilt. Subject data, however, reveal that none of the vibratory asymmetry measures correlates with spectral tilt measures. Probing further into physiological correlates of spectral tilt that might be affected by asymmetry, the glottal area waveform is parameterized to obtain measures of the open phase (open/plateau quotient) and closing phase (speed/closing quotient). Subjects' left-right phase asymmetry exhibits low, but statistically significant, correlations with speed quotient (r=0.45) and closing quotient (r=-0.39). Results call for future studies into the effect of asymmetric vocal fold vibration on glottal airflow and the associated impact on voice source spectral properties and vocal efficiency.     

Relationship among glottal area, static supraglottic compression, and laryngeal function studies in unilateral vocal fold paresis and paralysis

In this study, we evaluated the relationship between laryngeal function measures and glottal gap ratio and normalized measures of supraglottic behaviors in patients with unilateral vocal fold paresis (UVFP). Thirty-one patients were found to have unilateral vocal fold paresis by videoendoscopy and laryngeal electromyography, and 13 controls participated in this study. Patients with UVFP demonstrated significantly larger glottal gap ratios (p = 0.016) than control subjects. The nonparalyzed or contralateral vocal fold was associated with significantly more static false vocal fold compression (p = 0.03) compared with the paralyzed vocal fold or with the controls. Patients with unilateral vocal fold paresis were divided into subgroups: those with normal or abnormal maximum phonation time, flow, or pressure measures. Smaller glottal gap ratios were identified in patients with normal maximum phonation times and flow measures. Greater false vocal fold activity was identified in unilateral vocal fold paresis patients with normal laryngeal function measures than in unilateral vocal fold paresis patients with abnormal measures. These findings suggest that some patients with documented unilateral paresis and glottal incompetence can compensate for vocal fold weakness such that their acoustic and aerodynamic measures are normal.     

In Vivo Quantification of the Intraglottal Pressure: Modal Phonation and Voice Onset

Intraglottal pressure is the driving force of vocal fold vibration. Its time course during the open phase of the vibratory cycle is essential in the mechanics of phonation, but measuring it directly is difficult and may hinder spontaneous voicing. However, it can be computed from the in vivo measured transglottal flow and glottal area (hence the air particle velocity) on the basis of the Bernoulli energy law and the interaction with the inertance of the vocal tract. As to sustained modal phonation, calculations are presented for the two possible shapes of glottal duct: convergent and divergent, including absolute calibration in order to obtain quantitative physical values. Whatever the glottal duct configuration, the calculations based on measured values of glottal area and air flow show that the integrated intraglottal pressure during the opening phase systematically exceeds that during the closing phase, which is the basic condition for sustaining vocal fold oscillation. The key point is that the airflow curve is skewed to the right relative to the glottal area curve. The skewing results from air compressibility and vocal tract inertance. The intraglottal pressure becomes negative during the closing phase. As to the soft (or physiological) voice onset, a similar approach shows that the integrated pressure differences (opening phase − closing phase) actually increase as the onset progresses, and this applies to the results based on Bernoulli's energy law as well as to those based on the interaction with the inertance of the vocal tract. Furthermore and similarly, the phase lead of the pressure wave with respect to the glottal opening progressively increases. The underlying explanation lies in the progressively increasing skewing of the airflow curve to the right with respect to the glottal area curve.     

A theoretical model of the pressure field arising from asymmetric intraglottal flows applied to a two-mass model of the vocal folds

A theoretical flow solution is presented for predicting the pressure distribution along the vocal fold walls arising from asymmetric flow that forms during the closing phases of speech. The resultant wall jet was analyzed using boundary layer methods in a non-inertial reference frame attached to the moving wall. A solution for the near-wall velocity profiles on the flow wall was developed based on a Falkner-Skan similarity solution and it was demonstrated that the pressure distribution along the flow wall is imposed by the velocity in the inviscid core of the wall jet. The method was validated with experimental velocity data from 7.5 times life-size vocal fold models, acquired for varying flow rates and glottal divergence angles. The solution for the asymmetric pressures was incorporated into a widely used two-mass model of vocal fold oscillation with a coupled acoustical model of sound propagation. Asymmetric pressure loading was found to facilitate glottal closure, which yielded only slightly higher values of maximum flow declination rate and radiated sound, and a small decrease in the slope of the spectral tilt. While the impact on symmetrically tensioned vocal folds was small, results indicate the effect becomes more significant for asymmetrically tensioned vocal folds.     

Model-based classification of nonstationary vocal fold vibrations

Classification of vocal fold vibrations is an essential task of the objective assessment of voice disorders. For historical reasons, the conventional clinical examination of vocal fold vibrations is done during stationary, sustained phonation. However, the conclusions drawn from a stationary phonation are restricted to the observed steady-state vocal fold vibrations and cannot be generalized to voice mechanisms during running speech. This study addresses the approach of classifying real-time recordings of vocal fold oscillations during a nonstationary phonation paradigm in the form of a pitch raise. The classification is based on asymmetry measures derived from a time-dependent biomechanical two-mass model of the vocal folds which is adapted to observed vocal fold motion curves with an optimization procedure. After verification of the algorithm performance the method was applied to clinical problems. Recordings of ten subjects with normal voice and ten dysphonic subjects have been evaluated during stationary as well as nonstationary phonation. In the case of nonstationary phonation the model-based classification into "normal" and "dysphonic" succeeds in all cases, while it fails in the case of sustained phonation. The nonstationary vocal fold vibrations contain additional information about vocal fold irregularities, which are needed for an objective interpretation and classification of voice disorders.     

Optimal glottal configuration for ease of phonation

Recent experimental studies have shown the existence of optimalvalues of the glottal width and convergence angle, at which the phonation threshold pressure is minimum. These results indicate the existence of an optimal glottal configuration for ease of phonation, not predicted by the previous theory. In this paper, the origin of the optimal configuration is investigated using a low dimensional mathematical model of the vocal fold. Two phenomena of glottal aerodynamics are examined: pressure losses due to air viscosity, and air flow separation from a divergent glottis. The optimal glottal configuration seems to be a consequence of the combined effect of both factors. The results agree with the experimental data, showing that the phonation threshold pressure is minimum when the vocal folds are slightly separated in a near rectangular glottis.     

Strobophotoglottographic transillumination as a method for the analysis of vocal fold vibration patterns

The purpose of this exploratory study was to determine if laryngeal transillumination in combination with stroboscopy (strobophotoglottography; SPGG) is useful for (1) the visualization of vocal fold vibration (VFV) opening patterns, (2) the localization of initial vocal fold opening in horizontal glottal thirds (anterior, midmembranous, and posterior), (3) determination of the temporal correspondence of the so-called electroglottography (EGG)-knee and initial vocal fold separation, and, finally, (4) automatized quantitative measurements of glottal area function within endoscopic images. With stroboscopic transillumination, initial inferior vocal fold separation was detectable during the "closed" phase, where the vocal folds were still closed in the upper portion and therefore initial inferior vocal fold separation could not be visualized with usual laryngoscopy techniques. In the horizontal plane within similar fundamental frequencies in modal voice registers in two male subjects, localization of initial glottal opening depended on the voice types used (soft, normal, or pressed phonation). We found zipperlike posterior-to-anterior openings, initial midmembranous openings, initial anterior openings, as well as simultaneous initial opening of all three portions in the two healthy male adults examined. This technique proved to add temporal and spatial information to vocal fold opening patterns and extends our examination techniques to the very beginning of vocal fold opening at the inferior portion. Simultaneous electroglottogram tracking and comparison with bidirectionally illuminated stroboscopic images revealed a time-locked correspondence of the EGG-knee with the aforementioned initial inferior vocal fold separation. Bidirectional illumination combined with digital color extraction techniques allowed for image separation of subglottally and supraglottally illuminated structures. This facilitated vocal fold contour detection and automatized image processing, for example, for determination of glottal area function, and is considered to be a further step to objective automatized quantitative measurements within endoscopic images.     

The membranous contact quotient: A new phonatory measure of glottal competence

The membranous contact quotient (MCQ) is introduced as a measure of dynamic glottal competence. It is defined as the ratio of the membranous contact glottis (the anterior-posterior length of contact between the two membranous vocal folds) and the membranous vocal fold length. An elliptical approximation to the vocal fold contour during phonation was used to predict MCQ values as a function of vocal process gap (adduction), maximum glottal width, and membranous glottal length. MCQ is highly dependent on the vocal process gap and the maximum glottal width, but not on vocal fold length. Five excised larynges were used to obtain MCQ data for a wide range of vocal process gaps and maximum glottal widths. Predicted and measured MCQ values had a correlation of 0.93, with an average absolute difference of 9.6% (SD = 10.5%). The model is better at higher values of MCQ. The theory for MCQ is also expressed as a function of vocal process gap and subglottal pressure to suggest production control potential. The MCQ measure is obtainable with the use of stroboscopy and appears to be a potentially useful clinical measure.     

Analysis of Voice and Quantitative Measurement of Glottal Gap After Thyroplasty Type I in the Treatment of Unilateral Vocal Paralysis

Thyroplasty type I is one of several surgical treatments in which improving the voice of unilateral vocal fold paralysis is the ultimate objective. The goal of the surgery is the medialization of the paralyzed vocal fold. The purpose of this study is to evaluate the effectiveness of thyroplasty type I through acoustical analysis, aerodynamic measures, and quantitative videostroboscopic measurements. We report on 20 patients with unilateral vocal cord paralysis who underwent thyroplasty type I. We performed preoperative and postoperative video image analysis (normalized glottal gap area) and computer-assisted voice analysis (fundamental frequency, jitter, shimmer, noise-to-harmonic ratio, mean phonation time, mean flow rate, mean subglottic pressure) in all patients. The glottal gap was significantly reduced after thyroplasty type I. Postoperative voice quality was characterized by an improved pitch and amplitude pertubation (jitter and shimmer), phonation time (mean phonation time), and subglottic pressure (mean subglottic pressure). Thyroplasty type I is an effective method for regaining glottal closure and vocal function.     

Glottal flow through a two-mass model: comparison of Navier-Stokes solutions with simplified models

A new numerical model of the vocal folds is presented based on the well-known two-mass models of the vocal folds. The two-mass model is coupled to a model of glottal airflow based on the incompressible Navier-Stokes equations. Glottal waves are produced using different initial glottal gaps and different subglottal pressures. Fundamental frequency, glottal peak flow, and closed phase of the glottal waves have been compared with values known from the literature. The phonation threshold pressure was determined for different initial glottal gaps. The phonation threshold pressure obtained using the flow model with Navier-Stokes equations corresponds better to values determined in normal phonation than the phonation threshold pressure obtained using the flow model based on the Bernoulli equation. Using the Navier-Stokes equations, an increase of the subglottal pressure causes the fundamental frequency and the glottal peak flow to increase, whereas the fundamental frequency in the Bernoulli-based model does not change with increasing pressure.     

On the difference between negative damping and eigenmode synchronization as two phonation onset mechanisms

Negative damping and eigenmode synchronization as two different mechanisms of phonation onset are distinguished. Although both mechanisms lead to a favorable phase relationship between the flow pressure and the vocal fold motion as required for a net energy transfer into the vocal folds, the underlying mechanisms for this favorable phase relationship are different. The negative damping mechanism relies on glottal aerodynamics or acoustics to establish before onset and maintain after onset the favorable phase relationship, and therefore has minimum requirements on vocal fold geometry and biomechanics. A single degree-of-freedom vocal fold model is all that is needed for self-oscillation in the presence of a negative damping mechanism. In contrast, the mechanism of eigenmode synchronization critically depends on the geometrical and biomechanical properties of the vocal folds (at least 2-degrees-of-freedom are required), and has little requirement on the glottal aerodynamics other than flow separation. The favorable phase relation is established once synchronization occurs, regardless of the phase relationship imposed by glottal aerodynamics before onset. Unlike that of the negative damping mechanism, initiation of eigenmode synchronization requires neither a velocity-dependent flow pressure nor an alternating convergent-divergent glottis. The clinical implications of the distinctions between these two mechanisms are discussed.     

Resonance properties of the vocal folds: in vivo laryngoscopic investigation of the externally excited laryngeal vibrations

The study presents the first attempt to investigate resonance properties of the living vocal folds by means of laryngoscopy. Laryngeal vibrations were excited via a shaker placed on the neck of a male subject and observed by means of videostroboscopy and videokymography (VKG). When the vocal folds were tuned to the phonation frequency of 110 Hz and sinusoidal vibration with sweeping frequency (in the range 50-400 Hz) was delivered to the larynx, three clearly pronounced resonance peaks at frequencies around 110, 170, and 240 Hz were identified in the vocal fold tissues. Different modes of vibration of the vocal folds, observed as distinct lateral-medial oscillations with one, two, and three half-wavelengths along the glottal length, respectively, were associated with these resonance frequencies. At the external excitation frequencies below 100 Hz, vibrations of the ventricular folds, aryepiglottic folds and arytenoid cartilages were dominant in the larynx.