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.

Principal dimensions of voice production and their role in vocal expression.

How we produce and perceive voice is constrained by laryngeal physiology and biomechanics. Such constraints may present themselves as principal dimensions in the voice outcome space that are shared among speakers. This study attempts to identify such principal dimensions in the voice outcome space and the underlying laryngeal control mechanisms in a three-dimensional computational model of voice production. A large-scale voice simulation was performed with parametric variations in vocal fold geometry and stiffness, glottal gap, vocal tract shape, and subglottal pressure. Principal component analysis was applied to data combining both the physiological control parameters and voice outcome measures. The results showed three dominant dimensions accounting for at least 50% of the total variance. The first two dimensions describe respiratory-laryngeal coordination in controlling the energy balance between low- and high-frequency harmonics in the produced voice, and the third dimension describes control of the fundamental frequency. The dominance of these three dimensions suggests that voice changes along these principal dimensions are likely to be more consistently produced and perceived by most speakers than other voice changes, and thus are more likely to have emerged during evolution and be used to convey important personal information, such as emotion and larynx size.

Amount and spatial arrangement of muscle fibers in the human laryngeal Musculus ventricularis.

Textbooks and atlases of human macroscopic and microscopic anatomy of the larynx generally provide, if at all, only sparse information on the laryngeal Musculus ventricularis. However, several studies indicate that this muscle takes over the function of vestibular (ventricular) fold phonation after denervation of the Musculus vocalis. In the present study, 29 laryngeal specimens were coronally dissected at different levels, i.e. the anterior (L1), middle (L2), and posterior third of the vestibular fold (L3), and they underwent histological analysis. In all specimens the vestibular folds of both hemi-larynxes contained striated muscle bundles in variable amounts, representing a ventricularis muscle. These muscle bundles obviously originated from the lateral (external) and thyroepiglottic part of the thyroarytenoid muscle and the aryepiglottic part of the oblique arytenoid muscle, as has been described by other authors. The areas of vestibular folds and their amounts of ventricularis muscle bundles were measured using image analysis software (imageJ) by manual tracing. The mean area of the vestibular folds of both hemi-larynxes was 27.9 mm2 (SD [standard deviation] ± 9.17), and the area occupied by fibers of the ventricularis muscle was 1.5 mm2 (SD ± 1.78). Statistical analysis comparing the areas of both hemi-larynxes and levels resulted in no significant differences, except for the levels 2 and 3. In level 2, significantly more muscle fibers (2.0 mm2 ; SD ± 2.21) were detectable within the vestibular fold than in level 3 (0.9 mm2 ; SD ± 1.43). Level 1 also contained more muscle fibers (1.1 mm2 ; SD ± 1.06) than level 3, however, without significance. In conclusion, the laryngeal ventricularis muscle is present in the majority of reported cases. Since the muscle is of clinical relevance, it should be included in anatomical textbooks by default.

Vocal tract allometry in a mammalian vocal learner.

Acoustic allometry occurs when features of animal vocalisations can be predicted from body size measurements. Despite this being considered the norm, allometry sometimes breaks, resulting in species sounding smaller or larger than expected for their size. A recent hypothesis suggests that allometry-breaking mammals cluster into two groups: those with anatomical adaptations to their vocal tracts and those capable of learning new sounds (vocal learners). Here, we tested which mechanism is used to escape from acoustic allometry by probing vocal tract allometry in a proven mammalian vocal learner, the harbour seal (Phoca vitulina). We tested whether vocal tract structures and body size scale allometrically in 68 young individuals. We found that both body length and body mass accurately predict vocal tract length and one tracheal dimension. Independently, body length predicts vocal fold length while body mass predicts a second tracheal dimension. All vocal tract measures are larger in weaners than in pups and some structures are sexually dimorphic within age classes. We conclude that harbour seals do comply with anatomical allometric constraints. However, allometry between body size and vocal fold length seems to emerge after puppyhood, suggesting that ontogeny may modulate the anatomy-learning distinction previously hypothesised as clear cut. We suggest that seals, and perhaps other species producing signals that deviate from those expected from their vocal tract dimensions, may break allometry without morphological adaptations. In seals, and potentially other vocal learning mammals, advanced neural control over vocal organs may be the main mechanism for breaking acoustic allometry.

Mechanisms of sound production in deer mice (Peromyscus spp.).

Rodent diversification is associated with a large diversity of species-specific social vocalizations generated by two distinct laryngeal sound production mechanisms: whistling and airflow-induced vocal fold vibration. Understanding the relative importance of each modality to context-dependent acoustic interactions requires comparative analyses among closely related species. In this study, we used light gas experiments, acoustic analyses and laryngeal morphometrics to identify the distribution of the two mechanisms among six species of deer mice (Peromyscus spp.). We found that high frequency vocalizations (simple and complex sweeps) produced in close-distance contexts were generated by a whistle mechanism. In contrast, lower frequency sustained vocalizations (SVs) used in longer distance communication were produced by airflow-induced vocal fold vibrations. Pup isolation calls, which resemble adult SVs, were also produced by airflow-induced vocal fold vibrations. Nonlinear phenomena (NLP) were common in adult SVs and pup isolation calls, suggesting irregular vocal fold vibration characteristics. Both vocal production mechanisms were facilitated by a characteristic laryngeal morphology, including a two-layered vocal fold lamina propria, small vocal membrane-like extensions on the free edge of the vocal fold, and a singular ventral laryngeal air pocket known as the ventral pouch. The size and composition of vocal folds (rather than total laryngeal size) appears to contribute to species-specific acoustic properties. Our findings suggest that dual modes of sound production are more widespread among rodents than previously appreciated. Additionally, the common occurrence of NLP highlights the nonlinearity of the vocal apparatus, whereby small changes in anatomy or physiology trigger large changes in behavior. Finally, consistency in mechanisms of sound production used by neonates and adults underscores the importance of considering vocal ontogeny in the diversification of species-specific acoustic signals.

Selection on vocal output affects laryngeal morphology in rats.

Although laryngeal morphology often reflects adaptations for vocalization, the structural consequences of selection for particular aspects of vocal behavior remain poorly understood. In this study, we investigated the effects of increased ultrasonic calling in pups on the adult larynx morphology in selectively bred rat lines. Laryngeal morphology was assessed using multiple techniques: mineralized cartilage volumes were compared in 3D-models derived from microCT scans, internal structure was compared using clearing and staining procedures combined with microscopy, cellular structure was compared using histology and microscopy, and element composition was assessed with scanning energy dispersive X-ray spectroscopy. Our results show that adult rats from lines bred to produce ultrasonic calls at higher rates as pups have shorter vocal folds and a more mineralized thyroid cartilage compared to rats bred to produce ultrasonic calls at lower rates. The change in vocal fold length appears to account for differences in low-frequency calls in these two rat lines. We suggest that the observed increases in mineralization of the thyroid cartilage in the high-ultrasound lineage provide increased reinforcement of the laryngeal structure during ultrasonic call production. Our findings therefore demonstrate an effect of selection for vocal behavior on laryngeal morphology, with acoustic consequences.

Savannah roars: The vocal anatomy and the impressive rutting calls of male impala (Aepyceros melampus) - highlighting the acoustic correlates of a mobile larynx.

A retractable larynx and adaptations of the vocal folds in the males of several polygynous ruminants serve for the production of rutting calls that acoustically announce larger than actual body size to both rival males and potential female mates. Here, such features of the vocal tract and of the sound source are documented in another species. We investigated the vocal anatomy and laryngeal mobility including its acoustical effects during the rutting vocal display of free-ranging male impala (Aepyceros melampus melampus) in Namibia. Male impala produced bouts of rutting calls (consisting of oral roars and interspersed explosive nasal snorts) in a low-stretch posture while guarding a rutting territory or harem. For the duration of the roars, male impala retracted the larynx from its high resting position to a low mid-neck position involving an extensible pharynx and a resilient connection between the hyoid apparatus and the larynx. Maximal larynx retraction was 108 mm based on estimates in video single frames. This was in good concordance with 91-mm vocal tract elongation calculated on the basis of differences in formant dispersion between roar portions produced with the larynx still ascended and those produced with maximally retracted larynx. Judged by their morphological traits, the larynx-retracting muscles of male impala are homologous to those of other larynx-retracting ruminants. In contrast, the large and massive vocal keels are evolutionary novelties arising by fusion and linear arrangement of the arytenoid cartilage and the canonical vocal fold. These bulky and histologically complex vocal keels produced a low fundamental frequency of 50 Hz. Impala is another ruminant species in which the males are capable of larynx retraction. In addition, male impala vocal folds are spectacularly specialized compared with domestic bovids, allowing the production of impressive, low-frequency roaring vocalizations as a significant part of their rutting behaviour. Our study expands knowledge on the evolutionary variation of vocal fold morphology in mammals, suggesting that the structure of the mammalian sound source is not always human-like and should be considered in acoustic analysis and modelling.

A Convolutional Neural Network for Real Time Classification, Identification, and Labelling of Vocal Cord and Tracheal Using Laryngoscopy and Bronchoscopy Video.

BACKGROUND: The use of artificial intelligence, including machine learning, is increasing in medicine. Use of machine learning is rising in the prediction of patient outcomes. Machine learning may also be able to enhance and augment anesthesia clinical procedures such as airway management. In this study, we sought to develop a machine learning algorithm that could classify vocal cords and tracheal airway anatomy real-time during video laryngoscopy or bronchoscopy as well as compare the performance of three novel convolutional networks for detecting vocal cords and tracheal rings. METHODS: Following institutional approval, a clinical dataset of 775 video laryngoscopy and bronchoscopy videos was used. The dataset was divided into two categories for use for training and testing. We used three convolutional neural networks (CNNs): ResNet, Inception and MobileNet. Backpropagation and a mean squared error loss function were used to assess accuracy as well as minimize bias and variance. Following training, we assessed transferability using the generalization error of the CNN, sensitivity and specificity, average confidence error, outliers, overall confidence percentage, and frames per second for live video feeds. After the training was complete, 22 models using 0 to 25,000 steps were generated and compared. RESULTS: The overall confidence of classification for the vocal cords and tracheal rings for ResNet, Inception and MobileNet CNNs were as follows: 0.84, 0.78, and 0.64 for vocal cords, respectively, and 0.69, 0.72, 0.54 for tracheal rings, respectively. Transfer learning following additional training resulted in improved accuracy of ResNet and Inception for identifying the vocal cords (with a confidence of 0.96 and 0.93 respectively). The two best performing CNNs, ResNet and Inception, achieved a specificity of 0.985 and 0.971, respectively, and a sensitivity of 0.865 and 0.892, respectively. Inception was able to process the live video feeds at 10 FPS while ResNet processed at 5 FPS. Both were able to pass a feasibility test of identifying vocal cords and tracheal rings in a video feed. CONCLUSIONS: We report the development and evaluation of a CNN that can identify and classify airway anatomy in real time. This neural network demonstrates high performance. The availability of artificial intelligence may improve airway management and bronchoscopy by helping to identify key anatomy real time. Thus, potentially improving performance and outcomes during these procedures. Further, this technology may theoretically be extended to the settings of airway pathology or airway management in the hands of experienced providers. The researchers in this study are exploring the performance of this neural network in clinical trials.

The remarkable vocal anatomy of the koala (Phascolarctos cinereus): insights into low-frequency sound production in a marsupial species.

Koalas are characterised by a highly unusual vocal anatomy, with a descended larynx and velar vocal folds, allowing them to produce calls at disproportionately low frequencies. Here we use advanced imaging techniques, histological data, classical macroscopic dissection and behavioural observations to provide the first detailed description and interpretation of male and female koala vocal anatomy. We show that both males and females have an elongated pharynx and soft palate, resulting in a permanently descended larynx. In addition, the hyoid apparatus has a human-like configuration in which paired dorsal, resilient ligaments suspend the hyoid apparatus from the skull, while the ventral parts tightly connect to the descended larynx. We also show that koalas can retract the larynx down into the thoracic inlet, facilitated by a dramatic evolutionary transformation of the ventral neck muscles. First, the usual retractors of the larynx and the hyoid have their origins deep in the thorax. Secondly, three hyoid muscles have lost their connection to the hyoid skeleton. Thirdly, the genioglossus and geniohyoid muscles are greatly increased in length. Finally, the digastric, omohyoid and sternohyoid muscles, connected by a common tendinous intersection, form a guiding channel for the dynamic down-and-up movements of the ventral hyoid parts and the larynx. We suggest that these features evolved to accommodate the low resting position of the larynx and assist in its retraction during call production. We also confirm that the edges of the intra-pharyngeal ostium have specialised to form the novel, extra-laryngeal velar vocal folds, which are much larger than the true intra-laryngeal vocal folds in both sexes, but more developed and specialised for low frequency sound production in males than in females. Our findings illustrate that strong selection pressures on acoustic signalling not only lead to the specialisation of existing vocal organs but can also result in the evolution of novel vocal structures in both sexes.

Distances from vocal cords to mid-trachea for optimizing endotracheal tubes depth markers according to gestational age.

BACKGROUND: Adequate endotracheal tube positioning in preterm infants is complicated by the short length of the airway. Distal markers were designed to help with the insertion of endotracheal tubes at an appropriate depth below the vocal cords. However, those markers are not standardized between manufacturers, each tube size displays only one (sometimes 2) markers to provide information for infants of various gestational ages, and indicated distances are often too long for extremely preterm infants. AIMS: The study aims to describe vocal cords to mid-tracheal distance for different gestational ages and determine if depth markers should be adjusted accordingly. METHODS: Half the tracheal length added to the height of the posterior lamina of the cricoids approximates the distance between vocal cords and mid-trachea. Those dimensions were retrospectively retrieved from a database of laryngo-tracheal measurements obtained during autopsies of fetuses and newborn infants free of upper airway malformations. The equation of correlation between gestational age and distance from vocal cords to mid-trachea was used to calculate those distances for different gestational ages. RESULTS: Data were derived from 114 patients. Vocal cords to mid-trachea distance is linearly correlated with gestational age (r = .90; distance = 2.831 + 0.6208 × gestational age). We suggest depth markers at 17.7, 19.0, 20.8, 22.7, 24.6, and 26.4 mm for gestational ages of 24, 26, 29, 32, 35, and 38 weeks, respectively, indicated by contrasting colors. CONCLUSION: The linear relationship between laryngo-tracheal size and gestational age offers the opportunity to revise endotracheal tube depth markers for the smallest patients. Trials comparing those suggested markers with those currently in use are needed before implementation.

An age-dependent vocal tract model for males and females based on anatomic measurements.

The purpose of this study was to take a first step toward constructing a developmental and sex-specific version of a parametric vocal tract area function model representative of male and female vocal tracts ranging in age from infancy to 12 yrs, as well as adults. Anatomic measurements collected from a large imaging database of male and female children and adults provided the dataset from which length warping and cross-dimension scaling functions were derived, and applied to the adult-based vocal tract model to project it backward along an age continuum. The resulting model was assessed qualitatively by projecting hypothetical vocal tract shapes onto midsagittal images from the cohort of children, and quantitatively by comparison of formant frequencies produced by the model to those reported in the literature. An additional validation of modeled vocal tract shapes was made possible by comparison to cross-sectional area measurements obtained for children and adults using acoustic pharyngometry. This initial attempt to generate a sex-specific developmental vocal tract model paves a path to study the relation of vocal tract dimensions to documented prepubertal acoustic differences.

Grasshopper mice employ distinct vocal production mechanisms in different social contexts.

Functional changes in vocal organ morphology and motor control facilitate the evolution of acoustic signal diversity. Although many rodents produce vocalizations in a variety of social contexts, few studies have explored the underlying production mechanisms. Here, we describe mechanisms of audible and ultrasonic vocalizations (USVs) produced by grasshopper mice (genus Onychomys). Grasshopper mice are predatory rodents of the desert that produce both loud, long-distance advertisement calls and USVs in close-distance mating contexts. Using live-animal recording in normal air and heliox, laryngeal and vocal tract morphological investigations, and biomechanical modelling, we found that grasshopper mice employ two distinct vocal production mechanisms. In heliox, changes in higher-harmonic amplitudes of long-distance calls indicate an airflow-induced tissue vibration mechanism, whereas changes in fundamental frequency of USVs support a whistle mechanism. Vocal membranes and a thin lamina propria aid in the production of long-distance calls by increasing glottal efficiency and permitting high frequencies, respectively. In addition, tuning of fundamental frequency to the second resonance of a bell-shaped vocal tract increases call amplitude. Our findings indicate that grasshopper mice can dynamically adjust motor control to suit the social context and have novel morphological adaptations that facilitate long-distance communication.

Predicting Achievable Fundamental Frequency Ranges in Vocalization Across Species.

Vocal folds are used as sound sources in various species, but it is unknown how vocal fold morphologies are optimized for different acoustic objectives. Here we identify two main variables affecting range of vocal fold vibration frequency, namely vocal fold elongation and tissue fiber stress. A simple vibrating string model is used to predict fundamental frequency ranges across species of different vocal fold sizes. While average fundamental frequency is predominantly determined by vocal fold length (larynx size), range of fundamental frequency is facilitated by (1) laryngeal muscles that control elongation and by (2) nonlinearity in tissue fiber tension. One adaptation that would increase fundamental frequency range is greater freedom in joint rotation or gliding of two cartilages (thyroid and cricoid), so that vocal fold length change is maximized. Alternatively, tissue layers can develop to bear a disproportionate fiber tension (i.e., a ligament with high density collagen fibers), increasing the fundamental frequency range and thereby vocal versatility. The range of fundamental frequency across species is thus not simply one-dimensional, but can be conceptualized as the dependent variable in a multi-dimensional morphospace. In humans, this could allow for variations that could be clinically important for voice therapy and vocal fold repair. Alternative solutions could also have importance in vocal training for singing and other highly-skilled vocalizations.

Sonographic Dynamic Description of the Laryngeal Tract: Definition of Quantitative Measures to Characterize Vocal Fold Motion and Estimation of Their Normal Values.

Vocal fold motion was analyzed during free breathing using two-dimensional dynamic ultrasound imaging. Two cadavers were first analyzed to define easily identifiable landmarks. Motion of the laryngeal tract was then analyzed in an axial plane. Left and right arytenoids and thyroid cartilage were defined on images corresponding to abduction and adduction of the laryngeal tract. Associated area measurements were established for 50 healthy subjects. All area indices were significantly larger during abduction than adduction. Symmetry of motion was established by comparing each hemi-larynx, and mobility fractions were defined. Normal values of laryngeal motion during free breathing were thus established.

Biaxial mechanical properties of human vocal fold cover under vocal fold elongation.

Mechanical properties of the human vocal fold cover layer were experimentally investigated in uniaxial and biaxial tensile tests. The results showed a coupling effect between the stress conditions along the anterior-posterior and transverse directions, with vocal fold elongation increasing vocal fold stiffness along both directions, thus allowing more efficient control of the fundamental frequency of voice through vocal fold elongation. This study also shows that vocal folds were nearly isotropic at resting conditions, thus a tendency to vibrate with incomplete glottal closure, but became increasingly anisotropic with increasing vocal fold elongation.

A finite element study on the cause of vocal fold vertical stiffness variation.

A finite element method based numerical indentation technique was used to quantify the effect of the material stiffness variation and the subglottal convergence angle of the vocal fold on the vertical stiffness difference of the medial surface. It was found that the vertical stiffness difference increased with the increasing subglottal angle, and it tended to saturate beyond a subglottal angle of about 50°. The material stiffness variation could be as important as the subglottal angle depending on the actual material properties.

An extended Kalman filter approach to non-stationary Bayesian estimation of reduced-order vocal fold model parameters.

The Bayesian framework for parameter inference provides a basis from which subject-specific reduced-order vocal fold models can be generated. Previously, it has been shown that a particle filter technique is capable of producing estimates and associated credibility intervals of time-varying reduced-order vocal fold model parameters. However, the particle filter approach is difficult to implement and has a high computational cost, which can be barriers to clinical adoption. This work presents an alternative estimation strategy based upon Kalman filtering aimed at reducing the computational cost of subject-specific model development. The robustness of this approach to Gaussian and non-Gaussian noise is discussed. The extended Kalman filter (EKF) approach is found to perform very well in comparison with the particle filter technique at dramatically lower computational cost. Based upon the test cases explored, the EKF is comparable in terms of accuracy to the particle filter technique when greater than 6000 particles are employed; if less particles are employed, the EKF actually performs better. For comparable levels of accuracy, the solution time is reduced by 2 orders of magnitude when employing the EKF. By virtue of the approximations used in the EKF, however, the credibility intervals tend to be slightly underpredicted.

Effect of level difference between left and right vocal folds on phonation: Physical experiment and theoretical study.

As an alternative factor to produce asymmetry between left and right vocal folds, the present study focuses on level difference, which is defined as the distance between the upper surfaces of the bilateral vocal folds in the inferior-superior direction. Physical models of the vocal folds were utilized to study the effect of the level difference on the phonation threshold pressure. A vocal tract model was also attached to the vocal fold model. For two types of different models, experiments revealed that the phonation threshold pressure tended to increase as the level difference was extended. Based upon a small amplitude approximation of the vocal fold oscillations, a theoretical formula was derived for the phonation threshold pressure. This theory agrees with the experiments, especially when the phase difference between the left and right vocal folds is not extensive. Furthermore, an asymmetric two-mass model was simulated with a level difference to validate the experiments as well as the theory. The primary conclusion is that the level difference has a potential effect on voice production especially for patients with an extended level of vertical difference in the vocal folds, which might be taken into account for the diagnosis of voice disorders.

Electrically conductive synthetic vocal fold replicas for voice production research.

A method of fabricating electrically conductive synthetic vocal fold replicas and monitoring their vibration via resistance measurement is presented. Normally non-conductive silicone replicas were coated with conductive graphite and subjected to long-term vibration tests. Synchronized resistance and imaging data using hemilarynx and full larynx configurations showed an inverse correlation between replica contact area and resistance during vibration, similar to clinical electroglottography (EGG) used to estimate vocal fold contact area. This method has potential for long-term replica vibration monitoring and studying basic physical relationships between resistance and contact area in vocal folds and vocal fold replicas.

An optical flow-based state-space model of the vocal folds.

High-speed movies of the vocal fold vibration are valuable data to reveal vocal fold features for voice pathology diagnosis. This work presents a suitable Bayesian model and a purely theoretical discussion for further development of a framework for continuum biomechanical features estimation. A linear and Gaussian nonstationary state-space model is proposed and thoroughly discussed. The evolution model is based on a self-sustained three-dimensional finite element model of the vocal folds, and the observation model involves a dense optical flow algorithm. The results show that the method is able to capture different deformation patterns between the computed optical flow and the finite element deformation, controlled by the choice of the model tissue parameters.