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    D. D. Mehta and R. E. Hillman, “The evolution of methods for imaging vocal fold phonatory function,” Perspectives on Speech Science and Orofacial Disorders, vol. 22, no. 1, pp. 5-13, 2012. Publisher's VersionAbstract
    Paper

    In this article, we provide a brief summary of the major technological advances that led to current methods for imaging vocal fold vibration during phonation including the development of indirect laryngoscopy, imaging of rapid motion, fiber optics, and digital image capture. We also provide a brief overview of new emerging technologies that could be used in the future for voice research and clinical voice assessment, including advances in laryngeal high-speed videoendoscopy, depth-kymography, and dynamic optical coherence tomography.

    D. D. Mehta, D. D. Deliyski, T. F. Quatieri, and R. E. Hillman, “Automated measurement of vocal fold vibratory asymmetry from high-speed videoendoscopy recordings,” Journal of Speech, Language, and Hearing Research, vol. 54, no. 1, pp. 47-54, 2011. Publisher's VersionAbstract
    Paper

    Purpose: In prior work, a manually derived measure of vocal fold vibratory phase asymmetry correlated to varying degrees with visual judgments made from laryngeal high-speed videoendoscopy (HSV) recordings. This investigation extended this work by establishing an automated HSV-based framework to quantify 3 categories of vocal fold vibratory asymmetry. Method: HSV-based analysis provided for cycle-to-cycle estimates of left-right phase asymmetry, left-right amplitude asymmetry, and axis shift during glottal closure for 52 speakers with no vocal pathology producing comfortable and pressed phonation. An initial cross-validation of the automated left-right phase asymmetry measure was performed by correlating the measure with other objective and subjective assessments of phase asymmetry. Results: Vocal fold vibratory asymmetry was exhibited to a similar extent in both comfortable and pressed phonations. The automated measure of left-right phase asymmetry strongly correlated with manually derived measures and moderately correlated with visual-perceptual ratings. Correlations with the visual-perceptual ratings remained relatively consistent as the automated measure was derived from kymograms taken at different glottal locations. Conclusions: An automated HSV-based framework for the quantification of vocal fold vibratory asymmetry was developed and initially validated. This framework serves as a platform for investigating relationships between vocal fold tissue motion and acoustic measures of voice function.

    D. D. Mehta, M. Zañartu, T. F. Quatieri, D. D. Deliyski, and R. E. Hillman, “Investigating acoustic correlates of human vocal fold vibratory phase asymmetry through modeling and laryngeal high-speed videoendoscopy,” The Journal of the Acoustical Society of America, vol. 130, pp. 3999-4009, 2011.Abstract
    Paper

    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.

    D. D. Mehta, D. D. Deliyski, S. M. Zeitels, T. F. Quatieri, and R. E. Hillman, “Voice production mechanisms following phonosurgical treatment of early glottic cancer,” Annals of Otology, Rhinology, and Laryngology, vol. 119, pp. 1-9, 2010. Publisher's VersionAbstract
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    Objectives: Although near-normal conversational voices can be achieved with the phonosurgical management of earlyglottic cancer, there are still acoustic and aerodynamic deficits in vocal function that must be better understood to helpfurther optimize phonosurgical interventions. Stroboscopic assessment is inadequate for this purpose.Methods: A newly developed color high-speed videoendoscopy (HSV) system that included time-synchronized recordingsof the acoustic signal was used to perform a detailed examination of voice production mechanisms in 14 subjects.Digital image processing techniques were used to quantify glottal phonatory function and to delineate relationships betweenvocal fold vibratory properties and acoustic perturbation measures.Results: The results for multiple measurements of vibratory asymmetry showed that 31% to 62% of subjects displayedhigher-than-normal average values, whereas the mean values for glottal closure duration (open quotient) and periodicityof vibration fell within normal limits. The average HSV-based measures did not correlate significantly with the acousticperturbation measures, but moderate correlations were exhibited between the acoustic measures and the SDs of the HSVbasedparameters.Conclusions: The use of simultaneous, time-synchronized HSV and acoustic recordings can provide new insights intopostoperative voice production mechanisms that cannot be obtained with stroboscopic assessment.

    D. D. Mehta and R. E. Hillman, “Voice assessment: Updates on perceptual, acoustic, aerodynamic, and endoscopic imaging methods,” Current Opinion in Otolaryngology & Head and Neck Surgery, vol. 16, pp. 211-215, 2008. Publisher's VersionAbstract
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    PURPOSE OF REVIEW: This paper describes recent advances in perceptual, acoustic, aerodynamic, and endoscopic imaging methods for assessing voice function. RECENT FINDINGS: We review advances from four major areas. PERCEPTUAL ASSESSMENT: Speech-language pathologists are being encouraged to use the new consensus auditory-perceptual evaluation of voice inventory for auditory-perceptual assessment of voice quality, and recent studies have provided new insights into listener reliability issues that have plagued subjective perceptual judgments of voice quality. ACOUSTIC ASSESSMENT: Progress is being made on the development of algorithms that are more robust for analyzing disordered voices, including the capability to extract voice quality-related measures from running speech segments. AERODYNAMIC ASSESSMENT: New devices for measuring phonation threshold air pressures and air flows have the potential to serve as sensitive indices of glottal phonatory conditions, and recent developments in aeroacoustic theory may provide new insights into laryngeal sound production mechanisms. ENDOSCOPIC IMAGING: The increased light sensitivity of new ultra high-speed color digital video processors is enabling high-quality endoscopic imaging of vocal fold tissue motion at unprecedented image capture rates, which promises to provide new insights into the mechanisms of normal and disordered voice production. SUMMARY: Some of the recent research advances in voice function assessment could be more readily adopted into clinical practice, whereas others will require further development.

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