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Voice research: So What? A clearer view of voice production, 25 years of progress; the speaking voice
Journal of Voice
Vol. 7, No. 4, pp. 293-300 © 1993 Raven Press, Ltd., New York
Voice Research: So What? A Clearer View of Voice Production, 25 Years of Progress; the Speaking Voice J o s e p h C. S t e m p l e Institute for Voice Analysis and Rehabilitation, Dayton, Ohio, U.S.A.
Summary: The past 25 years has yielded an impressive growth in our knowledge of vocal function. Interdisciplinary research cooperation in areas of laryngeal histology, vocal aerodynamics and acoustics, vocal fold vibratory characteristics, neurolaryngology, and phonatory models has led to a clearer view of voice production. This article offers a brief review of the progress that has been made in our understanding of the speaking voice and relates this knowledge to clinical practice. The importance of utilizing voice research to confirm traditional management techniques and to develop new physiologically based management approaches is also stressed. Key Words: Vocal function-Histology--Acoustic, aerodynamic, vibratory characteristics--Neurolaryngology--Phonatory models.
and function and their relation to performance." (page vi). Twenty years later, following an extensive review of approximately 60 years of voice therapy literature, I have also come to the conclusion that the more things change, the more they appear to stay the same. The 20 years has indeed yielded tremendous growth in our knowledge and understanding of laryngeal function and of normal and pathological phonation. However, the direct clinical management techniques used daily with voicedisordered patients have not been tested by the newly acquired scientific procedures, instruments, and knowledge. The fact that many of the same voice therapy techniques have remained in use for many years is not necessarily an indictment of present-day voice management. It is, rather, more a statement of strong appreciation and admiration for the clinicians of our early history. The accuracy of their empirical observation regarding voice function was uncanny and the worthiness of many of their management techniques has been well established by positive clinical results (2). However, I would suggest that the means to scientifically test the efficacy
A clearer view of voice production has evolved through advances in technology and research over the past 25 years. My personal interest in voice research is from a clinical perspective, with my interest in voice science directed at the questions, "what does this information mean to me clinically? How can the information help in the diagnosis and management of voice-disordered patients?" Therefore, this discussion regarding the advances in the knowledge of vocal function is guided by a clinical bias. Twenty years ago, in the third edition of her text, The voice and its disorders, Margaret Greene (1) stated, "The basic principles of voice training remain unchallenged despite newly coined terms of reference, such as airflow rate, since they are established on the unchanging anatomy and physiology of the human instrument. What is new is the knowledge recently acquired concerning structure
Accepted September 25, 1992. Address correspondence and reprint requests to Dr. J. C. Stemple, Institute for Voice Analysis and Rehabilitation, 369 W. First St. #408, Dayton, OH 45402, U.S.A. This work was presented at the 21 st annual symposium of the Voice Foundation, Philadelphia, June 17, 1992.
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of our management techniques and to develop new direct management approaches are upon us. INTERDISCIPLINARY COOPERATION So what have the last 25 years contributed to our clearer view of the speaking voice? One of the most important factors responsible for advancing our knowledge of the mechanisms of voice has been the growth of interdisciplinary research in the subject area. Speech and voice scientists, engineers, linguists, speech pathologists, phoniatricians, otolaryngologists, voice teachers, vocal coaches, neurologists, psychologists, physicists, physiologists, and computer scientists are some of the many professionals who have demonstrated an interest in basic, applied, and clinical research in voice function. Organizations such as the Voice Foundation, the International Association of Logopedics and Phoniatrics (IALP), the American Speech-Language Hearing Association, the American Academy of Otolaryngology, and the Acoustical Society of America, to name but a few, have encouraged and supported this interdisciplinary activity. The cooperation among disciplines has accelerated research activities and knowledge on several fronts. In the past 25 years, several major areas of research have included vocal fold histology, vocal aerodynamics, vocal acoustics, vibratory characteristics of the vocal folds, neurolaryngology, phonatory models, and evaluation/management of the disordered voice. Let us briefly examine some of the activities in each of these areas of research.
Vocal fold histology In an informal poll of voice scientists that I conducted while preparing this article, it was generally agreed that more than any other scientific advancement, Hirano's (3) description of the vocal fold structure accelerated our understanding of normal and pathological voice production. Using histological techniques, Hirano demonstrated the layered structure of the vocal folds. He distinguished five layers, including the epithelium, the superficial layer of the lamina propria, the intermediate layer of the lamina propria, the deep layer of the lamina propria, and the vocalis muscle. The top four layers are known as the mucous membrane and are controlled passively during vibration. The mechanical properties of the vocalis muscle are regulated both passively and actively. From a mechanical point of Journal of Voice, Vol. 7, No. 4, 1993
view the five layers may be reclassified into three sections. These include the cover: the epithelium and superficial layer of the lamina propria (passive); the transition: the intermediate and deep layers of the lamina propria (passive); and the body: the vocalis muscle (active and passive). The superficial layer of the lamina propria is known as Reinke's space. The intermediate and deep layers of the lamina propria comprise the vocal ligament. Understanding of the layered structure of the vocal folds has permitted us to make functional care and management decisions that benefit normal voice production. For example, we no longer "strip" vocal folds. Great care is taken to preserve and maintain the integrity of the mucous membrane, as even a minor irregularity may affect voice production. We attempt to minimize ingested chemical pollutants in the form of certain drugs, chemicals such as caffeine and alcohol, because of potential damage caused by dehydration and vascular problems. We inform patients of the ill effects of inhaled air pollutants such as direct or passive tobacco smoke and other inspired chemicals and fumes. We are now very aware of the importance of appropriate hydration as related to normal vocal fold function and encourage appropriate liquid intake. Of all animals studied, the layered structure of the vocal folds in humans has been found to be the most suitable for vocal fold vibration (4). The most pliable portion of the vocal fold is the most superficial layer, resulting in good mechanical impedance matching for vibratory movement. Histological studies have also demonstrated that the maturation of the vocal musculature is rather late, while aging symptoms appear early (5). Though changes in vocal fold structure vary greatly among individuals, general structural aging is characterized by thickening and increased edema of the superficial layer, thinning of the intermediate layer, and thickening of the deep layer. The epithelium demonstrates no marked changes. All of these vocal fold structural changes are less marked in females but as both sexes age, all muscles tend to become somewhat atrophied (5). Histopathological studies have demonstrated that the most common causes of laryngeal pathologic conditions are factors that increase the vascularity of the mucous membrane (6). These factors may include voice abuse, smoking, and infection, all of which make the mucosa more vulnerable to vocal trauma.
VOICE R E S E A R C H : SO WHAT?
Histological studies have been very important in demonstrating the differences in vocal fold structures as related to sex, age, and the age continuum and structure changes caused by voice abuse and misuse. Knowledge gained from this research has permitted the voice clinician to better understand and thus treat some of the mechanical causes of voice disorders. In addition, our expectations of voice improvement following various treatments such as radiation therapy and vocal fold surgery have been modified. Clinicians may also be more realistic in their expectations for improvement of voice disorders caused by laryngeal aging in the elderly population. Aerodynamic studies Histological studies have demonstrated the structure of the vocal folds. However, without an energy source the structure would simply sit as a mass: form without function. The driving force, of course, is the air pressure generated by the respiratory system. Normal control of inspiration and expiration is necessary to support adequate phonation. Assessment of the movements of the thorax and abdomen during speech production may be accomplished utilizing instruments such as mercury-filled strain gauges, a plethysmograph, and magnetometers (713). Aerodynamic measures that have proven useful in the assessment of voice function include airflow volume, airflow rate, air pressure, and resistance. Ohm's law demonstrates that each of these measures has a direct relationship with the other in that pressure equals flow times resistance, flow equals pressure/resistance, and resistance equals pressure/ flow. Hirano (14) demonstrated that, during normal speech, airflow rates may range between 50-200 ml/ s, with men producing higher rates than women. Airflow rate measures have proven clinically valuable. When closure of the vocal folds is compromised by mass lesion, poor muscular control or weakness, or neural problems, airflow rates will be greater than normal. Hyperadduction of the vocal folds will produce the opposite effect or less-thannormal airflow rates. Airflow rate measures, therefore, aid the voice clinician in understanding how the vocal folds are approximating, thus helping the clinician plan the appropriate management techniques. An increase of air pressure beneath the vocal folds (subglottic air pressure) is necessary to initiate
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and maintain phonation. Shipp and McGlone (15) demonstrated normal subglottic air pressure for conversational voice to be between 3 and 7 cm of H20. Subglottic air pressure measures are clinically significant, as higher than normal pressure levels may indicate inefficient vocal fold approximation or excessive lung pressures. Subglottic pressure estimates are now clinically possible, as Rothenberg (16) and Smitheran and Hixon (17) demonstrated the close relationship between intraoral pressure measures and subglottic pressure measures. Aerodynamic measures have proven useful in describing laryngeal function of patients with laryngeal pathologic conditions. Prior to these more objective measures, clinicians depended on less robust correlates to determine the status of phonatory aerodynamics. These methods included documentation of maximum sustained phonation, counting the number of syllables per breath, visual observation of respiratory type and effort, and calculation of the s/z ratio (18). These subjective methods, though somewhat useful, do not account for individual flow or volume variations, nor do they include the intensity and frequency monitoring during the measurement task that is necessary to yield meaningful clinical data (19). Acoustic studies The product of the dynamic interaction of airflow and the vocal fold mass is acoustic energy. Kent and Read (20) credit the digital computer for progress in the study of speech acoustics. Computing capability allows simultaneous digital recording of multiple signals for subsequent automated or interactive analysis. The computer has made acoustic data available and of practical use in the voice clinic. The reality of a busy voice clinic dictates that information must be quickly accessed for that information to be useful either diagnostically or therapeutically. Dedicated microcomputer instrumentation and software are available for this purpose. With this instrumentation, acoustic data may be taken for each patient and compared to available normative data or simply to data from the same patient following treatment. Acoustic measures have decreased the level of subjectivity inherent in voice testing. Some of the acoustic measures that have proven to be clinically useful are mean fundamental frequency: vibrating rate per second of the vocal folds (100-150 Hz, men; 180-250 Hz, women) (21); frequency variability: standard deviation of the fundamental freJournal of Voice, Vol. 7, No. 4, 1993
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quency over a reasonably long time (two to four semitones, male and female) (22); frequency range: entire range of frequencies an individual can produce (three octaves, young adults male or female, decreasing with age) (22); frequency perturbation (jitter): irregularity of vibration of the vocal folds (normative data dependent on method used to extract measure) (23-27); mean sound pressure level (SPL): average sound pressure of the voice signal measured in decibels (conversational speech exhibits SPL between 75-80 dB) (22); amplitude variability: amplitude of speaking that varies depending on the sounds spoken and the message (28). The variability of amplitude is expressed as a standard deviation of the mean SPL. The dynamic range is the range of vocal intensities a person can produce (minimum intensity, 50 dB; maximum intensity, 115 dB) (29). Amplitude perturbation (shimmer) is the variation of vocal fold amplitude from cycle to cycle (30-32). The Union of European Phoniatricians has recommended a standard method of evaluating voice production utilizing a method known as phonetography (33). Utilizing the acoustic measures of frequency and intensity, a phonetogram is created that defines an individual's vocal limits. Phonetograms are generated by asking patients to produce their loudest and softest voices at each half-tone step covering their entire pitch range. The data gathered are plotted on the phonetogram, where frequency is displayed on the horizontal axis and intensity on the vertical axis. It is purported that information gained from the phonetogram can help identify the artistic potential of voice, can be used as a diagnostic aid with dysphonic patients, can be used to monitor the results of treatment, and can be used to monitor the effectiveness of voice training (34). Recently, the Voice Committee of the IALP suggested that the term "phonetogram" was misleading and suggested instead use of the terminology "voice range profile" (34). Widely used in Europe, the voice range profile is not well known by clinicians in the United States but well worth exploring as a standard method of voice analysis. Vibratory characteristics Another major advance in the study of voice function has been in the description of vibratory characteristics of both normal and pathological voices. Methods such as electroglottography (35), photoglottography (36,37), and inverse filtering of Journal of Voice, Vol. 7, No. 4, 1993
the voice signal (38,28) have aided our understanding of normal vibration patterns. These methods have permitted indirect, relatively noninvasive observation of vocal fold vibratory patterns. According to Baken (22), the value of these techniques to the researcher and the clinician "lies in their ability to provide a close-up view of the relatively fine details of the movement and contact patterns of the vocal folds themselves" (page 198). While these measures are qualitative in nature and based on subjective interpretations, they have provided clinicians with a means of indirectly observing vocal fold movement during the course of therapy without having to rely on direct visualization. These observations aid clinicians in plotting the progress of therapy while the visual displays provided by the techniques may be used as feedback for the patient during the course of therapy (39). Direct observation of the vocal folds has been available since Garcia created the indirect laryngoscope in 1854. In 1965, Luchsinger and Arnold (40) made a strong case for the use of laryngostroboscopy. These authors stated that laryngostroboscopic examination of the vocal folds was indispensable for comprehensive laryngological diagnosis. Extensively used in Europe for many years, stroboscopic examination of the larynx has finally increased use in the diagnosis and treatment of voice disorders in the United States. The principle of the laryngeal stroboscope is quite simple. When the scope flashes a light equal to the frequency of the vibrating vocal folds, the folds will appear to be immobile since they are illuminated at the same phase of their vibratory cycle. When the light flashes at a slightly slower vibratory rate than the vocal folds, then they are illuminated at different phases in their cycle. The effect is a simulated, slow-motion view of the vocal fold vibration. Stroboscopy adds an entirely new dimension to observational study of phonation and diagnosis of laryngeal pathologic conditions. Indirect laryngoscopy and fiberoptic views of vocal fold movement are limited to observations regarding adduction and abduction of the folds. Stroboscopic observation permits a detailed description of several vibratory variables, including degree of glottal closure, vertical level of fold approximation, amplitude of vibration, status of the mucosal wave, phase closure, phase symmetry, and periodicity of vibration (41). Observation of these vibratory variables has vastly expanded the clinician's ability to understand
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VOICE RESEARCH: SO WHAT? normal and pathological vocal fold function, thus leading to improved diagnosis and treatment of voice disorders. Stroboscopy may be performed with either a mirror, flexible fiberoptic scope, or rigid transoral scope. When combined with videorecording equipment, laryngeal stroboscopy permits documentation of pretreatment and posttreatment of vocal fold condition and may be well utilized for patient education.
Neurolaryngology Neurolaryngology is a term that has recently become popular, especially with the development of new alternative surgical treatments for vocal fold paralysis and with the increase of the understanding of and t r e a t m e n t f o r s p a s m o d i c d y s p h o n i a . Faaborg-Andersen (42) was one of the first to use electromyography studies of laryngeal musculature. Electromyographic recordings of laryngeal muscle activity provide useful information about the physiology of specific muscles during phonation (43). Electromyography has become useful clinically for identifying paretic conditions of the vocal folds and for monitoring the success of nerve anastomosis, graphs, and transplants. Electromyography is also widely used to guide Botox injections for spasmodic dysphonia patients (44). As the topics of recent neurolaryngology conferences demonstrate, neurolaryngology is a new subspecialty area that is in the process of defining its goals. Phonatory models As previously described, Hirano's (3) body-cover model of phonation accelerated our understanding of voice production. Other anatomical and functional models have also expanded our understanding of normal phonation, with less information presently gained regarding pathological phonation (4552). From the two-mass model (53) that described the lateral-medial unidirectional movement of the vocal folds, computer and physical modeling is ever expanding and now includes complex models that describe vocal fold vibration in multidirectional terms, adding vertical and longitudinal dimensions to the lateral-medial movements (54,55). In addition to the intrinsic movements of the vocal folds during vibration, models have also described the extrinsic influences of the vocal tract (56,57,58). Models of phonation utilizing animal studies, physical models, and computer simulations have provided much understanding regarding normal
phonation. Continued research is necessary to develop effective models that will deal with pathological variations of phonation. Phonatory models that deal with abnormal vibratory patterns would lead to a greater understanding of how, why, and to what level of consistency various laryngeal pathologies produce abnormal voice qualities. This information may then have the potential of leading us toward more effective treatment strategies.
Evaluation techniques Perhaps the most significant advancement in my own clinical practice has been in the area of evaluation techniques. When I began practice more than 15 years ago, the traditional voice evaluation consisted of an indirect mirror examination performed by an otolaryngologist, a detailed history of the problem, and a subjective voice evaluation. The subjective voice evaluation included a statement of the voice quality, and a scaled description of voice parameters (loudness, pitch, pitch range, resonance, rate, and so on), as well as the s/z ratio and measurement of phonation time. Management progress was determined by comparing pretherapy and posttherapy tape recordings, the patient's report of progress, rescaling of the voice parameters, and judgments made by my own clinical ear. With the advances made possible by the past 25 years of voice research my clinical voice evaluation now includes indirect laryngeal examination, detailed history/patient interview, subjective voice evaluation, laryngeal videostroboscopy, acoustic analysis, and aerodynamic analysis. The latter three procedures add objective measures that may be directly related to the pathophysiology of laryngeal behavior. The objective measures serve as a check and balance to the subjective measures, the patient's report, and my own clinical ear. While no single test or single measure permit a total picture of vocal function, the combination of these diagnostic procedures provides a composite picture of how the laryngeal, respiratory, resonatory, and psychological systems are working together to produce voice. VOICE THERAPY AND THE FUTURE In May 1977, Paul Moore, writing in the Journal of Speech and Hearing Disorders, said, "Voice disorders are now in the what's there and how much stage. Descriptions and measurements of many kinds are accumulating. Before long, it should be Journal of Voice, Vol. 7, No. 4, 1993
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possible to develop mathematical models which will permit manipulation of variables and predictions of outcomes. Such information should provide a solid foundation for rational voice therapy." (59, page 159). Indeed, as this review has indicated, measurements of many kinds have accumulated that have greatly expanded understanding of vocal function. However, as stated in the introduction of this article, current voice therapy techniques have not been examined and tested scientifically utilizing new procedures, instrumentation, and knowledge. Many of the management techniques presently utilized were devised in the 1930s and 1940s from drills and exercises borrowed from voice and diction manuals and modified to improve the disordered voice (59). Many of these management techniques were and remain creative and effective, but are not necessarily based on scientific principles. Other management techniques that are continually promoted through the clinical literature, such as direct pitch modification for vocal hyperfunction cases or effort closure exercises used for unilateral vocal fold paralysis patients, may provide dubious results and need to be carefully examined. From the early foundation of voice rehabilitation have arisen several general management orientations or philosophies. They are based primarily on the clinician's biases and the mindset that directs his or her management focus. For the sake of discussion, I will classify these philosophical orientations as symptomatic voice therapy, psychogenic voice therapy, etiologic voice therapy, and eclectic voice therapy. In short, symptomatic voice therapy focuses on modification of deviant vocal symptoms such as breathiness, low pitch, hard glottal attacks, and so on as a means of improving voice quality. The focus of psychogenic voice therapy is on the emotional and psychosocial status of the patient that led to and maintained the voice disorder. Etiologic voice therapy concentrates on discovering the causes of the voice disorder and modification/elimination of these causes to improve the vocal condition and voice quality. The eclectic approach to voice therapy is the combination of any and all of these orientations to affect positive vocal change. It is my opinion that the advances in our knowledge of vocal function developed over the past 25 years have led us toward a new philosophical orientation of voice therapy, an orientation based on the expanded knowledge of how the voice is produced. The name I would suggest for this orientaJournal of Voice, Vol. 7, No. 4, 1993
tion is physiologic voice therapy. Future research directed at clinical methods should utilize knowledge of vocal function to develop voice therapy techniques that provide direct, effective, and efficient vocal function exercises similar to the physical therapy utilized for other parts of the body. Physiologic vocal function exercises should be developed to modify and improve laryngeal muscle strength, tone, balance, and stamina and improve the balance among laryngeal muscle effort, respiratory effort and control, and supraglottic modification of the laryngeal tone. I would challenge the clinical researchers to move toward a more objective physiologic approach to voice therapy. Much has been learned regarding vocal function during the past 25 years. We now know much more about "what's there and how much." Histology, acoustic, aerodynamic, and vibratory characteristics studies as well as phonatory models of voice production have all contributed to this knowledge. I believe that the knowledge gained has improved the ability to diagnose and treat voice disorders as well as enhance normal vocal production. The need to test the voice therapy approaches and to develop a scientific basis for the management techniques used by voice pathologists remains. As a clinician, I am enthusiastic about what research has taught us about vocal function during the past 25 years. The knowledge gained and the evaluation techniques learned have improved my clinical skills. Questions remain, however, regarding the utilization of this knowledge. For example, is the scientific information regarding vocal function developed adequately enough to be clinically useful? Is it premature to promote instrumental vocal function analysis in the clinical setting? Has the knowledge gained improved evaluation and management of voice disorders or is it simply overkill for the clinical process? I look forward to further discussions regarding these issues.
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Voice Research: So What? A Clearer View of Voice Production, 25 Years of Progress; the Speaking Voice J o s e p h C. S t e m p l e Institute for Voice Analysis and Rehabilitation, Dayton, Ohio, U.S.A.
Summary: The past 25 years has yielded an impressive growth in our knowledge of vocal function. Interdisciplinary research cooperation in areas of laryngeal histology, vocal aerodynamics and acoustics, vocal fold vibratory characteristics, neurolaryngology, and phonatory models has led to a clearer view of voice production. This article offers a brief review of the progress that has been made in our understanding of the speaking voice and relates this knowledge to clinical practice. The importance of utilizing voice research to confirm traditional management techniques and to develop new physiologically based management approaches is also stressed. Key Words: Vocal function-Histology--Acoustic, aerodynamic, vibratory characteristics--Neurolaryngology--Phonatory models.
and function and their relation to performance." (page vi). Twenty years later, following an extensive review of approximately 60 years of voice therapy literature, I have also come to the conclusion that the more things change, the more they appear to stay the same. The 20 years has indeed yielded tremendous growth in our knowledge and understanding of laryngeal function and of normal and pathological phonation. However, the direct clinical management techniques used daily with voicedisordered patients have not been tested by the newly acquired scientific procedures, instruments, and knowledge. The fact that many of the same voice therapy techniques have remained in use for many years is not necessarily an indictment of present-day voice management. It is, rather, more a statement of strong appreciation and admiration for the clinicians of our early history. The accuracy of their empirical observation regarding voice function was uncanny and the worthiness of many of their management techniques has been well established by positive clinical results (2). However, I would suggest that the means to scientifically test the efficacy
A clearer view of voice production has evolved through advances in technology and research over the past 25 years. My personal interest in voice research is from a clinical perspective, with my interest in voice science directed at the questions, "what does this information mean to me clinically? How can the information help in the diagnosis and management of voice-disordered patients?" Therefore, this discussion regarding the advances in the knowledge of vocal function is guided by a clinical bias. Twenty years ago, in the third edition of her text, The voice and its disorders, Margaret Greene (1) stated, "The basic principles of voice training remain unchallenged despite newly coined terms of reference, such as airflow rate, since they are established on the unchanging anatomy and physiology of the human instrument. What is new is the knowledge recently acquired concerning structure
Accepted September 25, 1992. Address correspondence and reprint requests to Dr. J. C. Stemple, Institute for Voice Analysis and Rehabilitation, 369 W. First St. #408, Dayton, OH 45402, U.S.A. This work was presented at the 21 st annual symposium of the Voice Foundation, Philadelphia, June 17, 1992.
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of our management techniques and to develop new direct management approaches are upon us. INTERDISCIPLINARY COOPERATION So what have the last 25 years contributed to our clearer view of the speaking voice? One of the most important factors responsible for advancing our knowledge of the mechanisms of voice has been the growth of interdisciplinary research in the subject area. Speech and voice scientists, engineers, linguists, speech pathologists, phoniatricians, otolaryngologists, voice teachers, vocal coaches, neurologists, psychologists, physicists, physiologists, and computer scientists are some of the many professionals who have demonstrated an interest in basic, applied, and clinical research in voice function. Organizations such as the Voice Foundation, the International Association of Logopedics and Phoniatrics (IALP), the American Speech-Language Hearing Association, the American Academy of Otolaryngology, and the Acoustical Society of America, to name but a few, have encouraged and supported this interdisciplinary activity. The cooperation among disciplines has accelerated research activities and knowledge on several fronts. In the past 25 years, several major areas of research have included vocal fold histology, vocal aerodynamics, vocal acoustics, vibratory characteristics of the vocal folds, neurolaryngology, phonatory models, and evaluation/management of the disordered voice. Let us briefly examine some of the activities in each of these areas of research.
Vocal fold histology In an informal poll of voice scientists that I conducted while preparing this article, it was generally agreed that more than any other scientific advancement, Hirano's (3) description of the vocal fold structure accelerated our understanding of normal and pathological voice production. Using histological techniques, Hirano demonstrated the layered structure of the vocal folds. He distinguished five layers, including the epithelium, the superficial layer of the lamina propria, the intermediate layer of the lamina propria, the deep layer of the lamina propria, and the vocalis muscle. The top four layers are known as the mucous membrane and are controlled passively during vibration. The mechanical properties of the vocalis muscle are regulated both passively and actively. From a mechanical point of Journal of Voice, Vol. 7, No. 4, 1993
view the five layers may be reclassified into three sections. These include the cover: the epithelium and superficial layer of the lamina propria (passive); the transition: the intermediate and deep layers of the lamina propria (passive); and the body: the vocalis muscle (active and passive). The superficial layer of the lamina propria is known as Reinke's space. The intermediate and deep layers of the lamina propria comprise the vocal ligament. Understanding of the layered structure of the vocal folds has permitted us to make functional care and management decisions that benefit normal voice production. For example, we no longer "strip" vocal folds. Great care is taken to preserve and maintain the integrity of the mucous membrane, as even a minor irregularity may affect voice production. We attempt to minimize ingested chemical pollutants in the form of certain drugs, chemicals such as caffeine and alcohol, because of potential damage caused by dehydration and vascular problems. We inform patients of the ill effects of inhaled air pollutants such as direct or passive tobacco smoke and other inspired chemicals and fumes. We are now very aware of the importance of appropriate hydration as related to normal vocal fold function and encourage appropriate liquid intake. Of all animals studied, the layered structure of the vocal folds in humans has been found to be the most suitable for vocal fold vibration (4). The most pliable portion of the vocal fold is the most superficial layer, resulting in good mechanical impedance matching for vibratory movement. Histological studies have also demonstrated that the maturation of the vocal musculature is rather late, while aging symptoms appear early (5). Though changes in vocal fold structure vary greatly among individuals, general structural aging is characterized by thickening and increased edema of the superficial layer, thinning of the intermediate layer, and thickening of the deep layer. The epithelium demonstrates no marked changes. All of these vocal fold structural changes are less marked in females but as both sexes age, all muscles tend to become somewhat atrophied (5). Histopathological studies have demonstrated that the most common causes of laryngeal pathologic conditions are factors that increase the vascularity of the mucous membrane (6). These factors may include voice abuse, smoking, and infection, all of which make the mucosa more vulnerable to vocal trauma.
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Histological studies have been very important in demonstrating the differences in vocal fold structures as related to sex, age, and the age continuum and structure changes caused by voice abuse and misuse. Knowledge gained from this research has permitted the voice clinician to better understand and thus treat some of the mechanical causes of voice disorders. In addition, our expectations of voice improvement following various treatments such as radiation therapy and vocal fold surgery have been modified. Clinicians may also be more realistic in their expectations for improvement of voice disorders caused by laryngeal aging in the elderly population. Aerodynamic studies Histological studies have demonstrated the structure of the vocal folds. However, without an energy source the structure would simply sit as a mass: form without function. The driving force, of course, is the air pressure generated by the respiratory system. Normal control of inspiration and expiration is necessary to support adequate phonation. Assessment of the movements of the thorax and abdomen during speech production may be accomplished utilizing instruments such as mercury-filled strain gauges, a plethysmograph, and magnetometers (713). Aerodynamic measures that have proven useful in the assessment of voice function include airflow volume, airflow rate, air pressure, and resistance. Ohm's law demonstrates that each of these measures has a direct relationship with the other in that pressure equals flow times resistance, flow equals pressure/resistance, and resistance equals pressure/ flow. Hirano (14) demonstrated that, during normal speech, airflow rates may range between 50-200 ml/ s, with men producing higher rates than women. Airflow rate measures have proven clinically valuable. When closure of the vocal folds is compromised by mass lesion, poor muscular control or weakness, or neural problems, airflow rates will be greater than normal. Hyperadduction of the vocal folds will produce the opposite effect or less-thannormal airflow rates. Airflow rate measures, therefore, aid the voice clinician in understanding how the vocal folds are approximating, thus helping the clinician plan the appropriate management techniques. An increase of air pressure beneath the vocal folds (subglottic air pressure) is necessary to initiate
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and maintain phonation. Shipp and McGlone (15) demonstrated normal subglottic air pressure for conversational voice to be between 3 and 7 cm of H20. Subglottic air pressure measures are clinically significant, as higher than normal pressure levels may indicate inefficient vocal fold approximation or excessive lung pressures. Subglottic pressure estimates are now clinically possible, as Rothenberg (16) and Smitheran and Hixon (17) demonstrated the close relationship between intraoral pressure measures and subglottic pressure measures. Aerodynamic measures have proven useful in describing laryngeal function of patients with laryngeal pathologic conditions. Prior to these more objective measures, clinicians depended on less robust correlates to determine the status of phonatory aerodynamics. These methods included documentation of maximum sustained phonation, counting the number of syllables per breath, visual observation of respiratory type and effort, and calculation of the s/z ratio (18). These subjective methods, though somewhat useful, do not account for individual flow or volume variations, nor do they include the intensity and frequency monitoring during the measurement task that is necessary to yield meaningful clinical data (19). Acoustic studies The product of the dynamic interaction of airflow and the vocal fold mass is acoustic energy. Kent and Read (20) credit the digital computer for progress in the study of speech acoustics. Computing capability allows simultaneous digital recording of multiple signals for subsequent automated or interactive analysis. The computer has made acoustic data available and of practical use in the voice clinic. The reality of a busy voice clinic dictates that information must be quickly accessed for that information to be useful either diagnostically or therapeutically. Dedicated microcomputer instrumentation and software are available for this purpose. With this instrumentation, acoustic data may be taken for each patient and compared to available normative data or simply to data from the same patient following treatment. Acoustic measures have decreased the level of subjectivity inherent in voice testing. Some of the acoustic measures that have proven to be clinically useful are mean fundamental frequency: vibrating rate per second of the vocal folds (100-150 Hz, men; 180-250 Hz, women) (21); frequency variability: standard deviation of the fundamental freJournal of Voice, Vol. 7, No. 4, 1993
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quency over a reasonably long time (two to four semitones, male and female) (22); frequency range: entire range of frequencies an individual can produce (three octaves, young adults male or female, decreasing with age) (22); frequency perturbation (jitter): irregularity of vibration of the vocal folds (normative data dependent on method used to extract measure) (23-27); mean sound pressure level (SPL): average sound pressure of the voice signal measured in decibels (conversational speech exhibits SPL between 75-80 dB) (22); amplitude variability: amplitude of speaking that varies depending on the sounds spoken and the message (28). The variability of amplitude is expressed as a standard deviation of the mean SPL. The dynamic range is the range of vocal intensities a person can produce (minimum intensity, 50 dB; maximum intensity, 115 dB) (29). Amplitude perturbation (shimmer) is the variation of vocal fold amplitude from cycle to cycle (30-32). The Union of European Phoniatricians has recommended a standard method of evaluating voice production utilizing a method known as phonetography (33). Utilizing the acoustic measures of frequency and intensity, a phonetogram is created that defines an individual's vocal limits. Phonetograms are generated by asking patients to produce their loudest and softest voices at each half-tone step covering their entire pitch range. The data gathered are plotted on the phonetogram, where frequency is displayed on the horizontal axis and intensity on the vertical axis. It is purported that information gained from the phonetogram can help identify the artistic potential of voice, can be used as a diagnostic aid with dysphonic patients, can be used to monitor the results of treatment, and can be used to monitor the effectiveness of voice training (34). Recently, the Voice Committee of the IALP suggested that the term "phonetogram" was misleading and suggested instead use of the terminology "voice range profile" (34). Widely used in Europe, the voice range profile is not well known by clinicians in the United States but well worth exploring as a standard method of voice analysis. Vibratory characteristics Another major advance in the study of voice function has been in the description of vibratory characteristics of both normal and pathological voices. Methods such as electroglottography (35), photoglottography (36,37), and inverse filtering of Journal of Voice, Vol. 7, No. 4, 1993
the voice signal (38,28) have aided our understanding of normal vibration patterns. These methods have permitted indirect, relatively noninvasive observation of vocal fold vibratory patterns. According to Baken (22), the value of these techniques to the researcher and the clinician "lies in their ability to provide a close-up view of the relatively fine details of the movement and contact patterns of the vocal folds themselves" (page 198). While these measures are qualitative in nature and based on subjective interpretations, they have provided clinicians with a means of indirectly observing vocal fold movement during the course of therapy without having to rely on direct visualization. These observations aid clinicians in plotting the progress of therapy while the visual displays provided by the techniques may be used as feedback for the patient during the course of therapy (39). Direct observation of the vocal folds has been available since Garcia created the indirect laryngoscope in 1854. In 1965, Luchsinger and Arnold (40) made a strong case for the use of laryngostroboscopy. These authors stated that laryngostroboscopic examination of the vocal folds was indispensable for comprehensive laryngological diagnosis. Extensively used in Europe for many years, stroboscopic examination of the larynx has finally increased use in the diagnosis and treatment of voice disorders in the United States. The principle of the laryngeal stroboscope is quite simple. When the scope flashes a light equal to the frequency of the vibrating vocal folds, the folds will appear to be immobile since they are illuminated at the same phase of their vibratory cycle. When the light flashes at a slightly slower vibratory rate than the vocal folds, then they are illuminated at different phases in their cycle. The effect is a simulated, slow-motion view of the vocal fold vibration. Stroboscopy adds an entirely new dimension to observational study of phonation and diagnosis of laryngeal pathologic conditions. Indirect laryngoscopy and fiberoptic views of vocal fold movement are limited to observations regarding adduction and abduction of the folds. Stroboscopic observation permits a detailed description of several vibratory variables, including degree of glottal closure, vertical level of fold approximation, amplitude of vibration, status of the mucosal wave, phase closure, phase symmetry, and periodicity of vibration (41). Observation of these vibratory variables has vastly expanded the clinician's ability to understand
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VOICE RESEARCH: SO WHAT? normal and pathological vocal fold function, thus leading to improved diagnosis and treatment of voice disorders. Stroboscopy may be performed with either a mirror, flexible fiberoptic scope, or rigid transoral scope. When combined with videorecording equipment, laryngeal stroboscopy permits documentation of pretreatment and posttreatment of vocal fold condition and may be well utilized for patient education.
Neurolaryngology Neurolaryngology is a term that has recently become popular, especially with the development of new alternative surgical treatments for vocal fold paralysis and with the increase of the understanding of and t r e a t m e n t f o r s p a s m o d i c d y s p h o n i a . Faaborg-Andersen (42) was one of the first to use electromyography studies of laryngeal musculature. Electromyographic recordings of laryngeal muscle activity provide useful information about the physiology of specific muscles during phonation (43). Electromyography has become useful clinically for identifying paretic conditions of the vocal folds and for monitoring the success of nerve anastomosis, graphs, and transplants. Electromyography is also widely used to guide Botox injections for spasmodic dysphonia patients (44). As the topics of recent neurolaryngology conferences demonstrate, neurolaryngology is a new subspecialty area that is in the process of defining its goals. Phonatory models As previously described, Hirano's (3) body-cover model of phonation accelerated our understanding of voice production. Other anatomical and functional models have also expanded our understanding of normal phonation, with less information presently gained regarding pathological phonation (4552). From the two-mass model (53) that described the lateral-medial unidirectional movement of the vocal folds, computer and physical modeling is ever expanding and now includes complex models that describe vocal fold vibration in multidirectional terms, adding vertical and longitudinal dimensions to the lateral-medial movements (54,55). In addition to the intrinsic movements of the vocal folds during vibration, models have also described the extrinsic influences of the vocal tract (56,57,58). Models of phonation utilizing animal studies, physical models, and computer simulations have provided much understanding regarding normal
phonation. Continued research is necessary to develop effective models that will deal with pathological variations of phonation. Phonatory models that deal with abnormal vibratory patterns would lead to a greater understanding of how, why, and to what level of consistency various laryngeal pathologies produce abnormal voice qualities. This information may then have the potential of leading us toward more effective treatment strategies.
Evaluation techniques Perhaps the most significant advancement in my own clinical practice has been in the area of evaluation techniques. When I began practice more than 15 years ago, the traditional voice evaluation consisted of an indirect mirror examination performed by an otolaryngologist, a detailed history of the problem, and a subjective voice evaluation. The subjective voice evaluation included a statement of the voice quality, and a scaled description of voice parameters (loudness, pitch, pitch range, resonance, rate, and so on), as well as the s/z ratio and measurement of phonation time. Management progress was determined by comparing pretherapy and posttherapy tape recordings, the patient's report of progress, rescaling of the voice parameters, and judgments made by my own clinical ear. With the advances made possible by the past 25 years of voice research my clinical voice evaluation now includes indirect laryngeal examination, detailed history/patient interview, subjective voice evaluation, laryngeal videostroboscopy, acoustic analysis, and aerodynamic analysis. The latter three procedures add objective measures that may be directly related to the pathophysiology of laryngeal behavior. The objective measures serve as a check and balance to the subjective measures, the patient's report, and my own clinical ear. While no single test or single measure permit a total picture of vocal function, the combination of these diagnostic procedures provides a composite picture of how the laryngeal, respiratory, resonatory, and psychological systems are working together to produce voice. VOICE THERAPY AND THE FUTURE In May 1977, Paul Moore, writing in the Journal of Speech and Hearing Disorders, said, "Voice disorders are now in the what's there and how much stage. Descriptions and measurements of many kinds are accumulating. Before long, it should be Journal of Voice, Vol. 7, No. 4, 1993
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possible to develop mathematical models which will permit manipulation of variables and predictions of outcomes. Such information should provide a solid foundation for rational voice therapy." (59, page 159). Indeed, as this review has indicated, measurements of many kinds have accumulated that have greatly expanded understanding of vocal function. However, as stated in the introduction of this article, current voice therapy techniques have not been examined and tested scientifically utilizing new procedures, instrumentation, and knowledge. Many of the management techniques presently utilized were devised in the 1930s and 1940s from drills and exercises borrowed from voice and diction manuals and modified to improve the disordered voice (59). Many of these management techniques were and remain creative and effective, but are not necessarily based on scientific principles. Other management techniques that are continually promoted through the clinical literature, such as direct pitch modification for vocal hyperfunction cases or effort closure exercises used for unilateral vocal fold paralysis patients, may provide dubious results and need to be carefully examined. From the early foundation of voice rehabilitation have arisen several general management orientations or philosophies. They are based primarily on the clinician's biases and the mindset that directs his or her management focus. For the sake of discussion, I will classify these philosophical orientations as symptomatic voice therapy, psychogenic voice therapy, etiologic voice therapy, and eclectic voice therapy. In short, symptomatic voice therapy focuses on modification of deviant vocal symptoms such as breathiness, low pitch, hard glottal attacks, and so on as a means of improving voice quality. The focus of psychogenic voice therapy is on the emotional and psychosocial status of the patient that led to and maintained the voice disorder. Etiologic voice therapy concentrates on discovering the causes of the voice disorder and modification/elimination of these causes to improve the vocal condition and voice quality. The eclectic approach to voice therapy is the combination of any and all of these orientations to affect positive vocal change. It is my opinion that the advances in our knowledge of vocal function developed over the past 25 years have led us toward a new philosophical orientation of voice therapy, an orientation based on the expanded knowledge of how the voice is produced. The name I would suggest for this orientaJournal of Voice, Vol. 7, No. 4, 1993
tion is physiologic voice therapy. Future research directed at clinical methods should utilize knowledge of vocal function to develop voice therapy techniques that provide direct, effective, and efficient vocal function exercises similar to the physical therapy utilized for other parts of the body. Physiologic vocal function exercises should be developed to modify and improve laryngeal muscle strength, tone, balance, and stamina and improve the balance among laryngeal muscle effort, respiratory effort and control, and supraglottic modification of the laryngeal tone. I would challenge the clinical researchers to move toward a more objective physiologic approach to voice therapy. Much has been learned regarding vocal function during the past 25 years. We now know much more about "what's there and how much." Histology, acoustic, aerodynamic, and vibratory characteristics studies as well as phonatory models of voice production have all contributed to this knowledge. I believe that the knowledge gained has improved the ability to diagnose and treat voice disorders as well as enhance normal vocal production. The need to test the voice therapy approaches and to develop a scientific basis for the management techniques used by voice pathologists remains. As a clinician, I am enthusiastic about what research has taught us about vocal function during the past 25 years. The knowledge gained and the evaluation techniques learned have improved my clinical skills. Questions remain, however, regarding the utilization of this knowledge. For example, is the scientific information regarding vocal function developed adequately enough to be clinically useful? Is it premature to promote instrumental vocal function analysis in the clinical setting? Has the knowledge gained improved evaluation and management of voice disorders or is it simply overkill for the clinical process? I look forward to further discussions regarding these issues.
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