1

Introduction

The main pitch control is achieved by the activity of the cricothyroid muscle during normal speech production or singing. The cricothyroid muscle changes the cricothyroid distance . This in turn leads to changes in the length of the vocal folds and then to changes in the longitudinal tension of the vocal folds. This muscle is composed of three distinct bellies, pars recta, pars oblique, and pars horizontalis, because of their orientations to the airway axis . The contraction of pars recta results in a decrease in the distance between the thyroid and cricoid cartilage by rotating the thyroid cartilage down toward the cricoid in the vertical axis . The pars oblique has an additional function of moving the thyroid cartilage forward and the cricoid cartilage backward . Thyroarytenoid muscle activity is shown to combine with cricothyroid muscle activity to regulate fundamental frequency of phonation. At lower fundamental frequencies and lower vocal intensities, pitch correlates positively with thyroarytenoid activity, but at higher fundamental frequencies and low intensity an increase in thyroarytenoid activity tends to lower pitch.

Neck muscles have been known to not only play an important role in swallowing but also have a significant influence on the voice during phonation. Unfortunately, there is not enough research on how laryngeal structures move dynamically in response to the movement of neck muscles. When the hyoid bone and the tongue are raised, the larynx tends to be pulled upward. The elevated larynx causes stiffening of the vocal fold margins perpendicular to the stiffness caused by the cricothyroid muscle activity, the so-called vertical tension. Hong et al. have reported the effect of neck muscles on frequency. They supposed that laryngeal elevation might result in increasing the vertical tension of the vocal folds with greater laryngotracheal pull. The increased vertical tension in the conus elasticus during laryngeal elevation causes an increase in the rate of vibration, and the decreased vertical tension during laryngeal lowering causes a decrease in the rate of vibration . Because the laryngeal height correlates positively with F0, this explanation seems very logical. However, many different explanations have been presented to explain the physiological background of intrinsic fundamental frequency (Fo) of vowels . When the larynx moves downward and the tongue moves upward, the mucosa of the larynx becomes thinner. Thus, this increased vertical tension of the vocal folds might especially affect the cover of the vocal folds. Honda and Fujimura suggested the possibility of longitudinal changes in the vocal folds as a cause of these F0 variations. They found that the EMG activity of the geniohyoid and the genioglossal muscles was positively correlated with the activity of the cricothyroid muscle and the intrinsic F0 of the vowels. It was also obvious that some change in the longitudinal tension of the vocal folds has to occur in order to explain the variation in intrinsic F0 of vowels found in vivo . This movement causes a vertical stretch of the hyoid-laryngeal tissue and an increase in the longitudinal tension.

With respect to the effect of laryngeal elevation on the pitch, Ohala reported the classical tongue pull hypothesis. He reported increased vertical tension of vocal folds after laryngeal elevation. The elevated larynx caused stiffening of the vocal fold margins perpendicular to the stiffness caused by the cricothyroid muscle activity. Erickson et al. reported an interesting finding that elevation of the larynx above the resting level provides a mechanical advantage for posterior rotation of the cricoid cartilage around the cricothyroid joint. They suggested that this vertical movement would apply forces in the same direction as the cricothyroid muscle activity, and therefore, the sum of these two forces determines the rotation of the cricoid cartilage. The importance of extralaryngeal frame function to the pitch control has been reported in patients with total thyroidectomy . The phonation time and fundamental frequency were not changed after surgery, but the speaking fundamental frequency (SFo), range of SFo and vocal range might be diminished after surgery. We suggested that the cause of voice dysfunction is not seen in a neural lesion, but in a disturbance of the extralaryngeal frame function. The purpose of this study was to evaluate the laryngeal dynamics during pitch changes. We evaluated the effect of the laryngeal elevation on pitch control. The movements of the hyoid bone, and thyroid and cricoid cartilages, and the cricothyroid distance were measured during pitch elevation and compared to the cricothyroid distance caused by the activity of the cricothyroid muscle.

2

Subjects and methods

Subjects were eight healthy adult males ( Table 1 ). Each subject underwent a comprehensive motor/structural examination prior to participation in this study. Subjects with any neurologic or structural abnormality affecting the head and neck were excluded. We reviewed an institutional review board approval and obtained proper consent from the normal subjects. Selection criteria for the subjects included clear visualization of the anterior–inferior edge of the hyoid bone, thyroid and cricoid cartilages by the conventional lateral neck films ( Fig. 1 ). For analyzing the pitch, Praat Voice Analysis Software (Version 5.3.61) was used. As a test method, the /ah/ sound in a natural voice was continuously increased from the low tone to the maximum.

Anterior and inferior regions of the 6th cervical vertebra were set as the origin “O” of the coordinate, and starting from the origin, the straight line connecting the anterior and inferior regions of the 2nd cervical vertebra was set as the y-axis. With the y-axis passing the origin as the center, the perpendicular line was set as the x-axis vertebra, and the x-axis was the line perpendicular to the y-axis. (A) Cricoid cartilage. (B). Thyroid cartilage. (C) Hyoid bone.

Video-fluoroscopic images are very useful for the analysis of laryngeal dynamics . The exposed dose of radiation to the skin during fluoroscopy is within permeable dose and no radiation risk. Radiographic magnification results in larger images of the distance between any two points, while radial distortion results in increasingly stretched images toward the periphery . These referents vary including under the chin, on the lateral neck, or the 2nd or 6th cervical vertebra as a calibration referent. In general, the 2nd and 6th cervical vertebrae are commonly used for vertically aligned reference axis in measurement of hyoid and laryngeal displacement. Video fluoroscopic image was analyzed by saving it in the computer at the speed of 30 frames per second in an Audio Video Interleave file, and voice was analyzed for its pitch every 0.03 seconds, thus matching the image with the voice. Subjects were seated upright in a stretcher chair and filmed with a mobile, C-arm X-ray system. The pitch was raised while the angle of the jaw did not increase. The angle range in all of the subjects’ was below 5°. During the test, they were asked to make an /ah/ sound from the lowest pitch to the highest pitch ( Table 1 ). The picture frames of video-fluoroscopic images (Shimadzu Corporation Sonialvision Versa 100I/DAR-8000, Japan) were then generated during pitch elevation. The hyoid bone, and cricoid and thyroid cartilages were in their resting position before phonation and these structures were maximally displaced, vertically and anteriorly, during pitch elevation. Resting position and maximum displacement of these structures were determined after observing the elevation of the larynx.

For analyzing the images, a motion analysis was conducted using Max TRAQ 2D Standard version 2.4.0.3 (Innovision systems, Inc.). This program includes tools such as angles, distances and scales. Analysis includes distances, angles, and center of mass. This program can be used to extract kinematic properties out of standard AVI files. We can go through frame by frame to look at angles and distance between points. To compensate for the cervical motion, anterior and inferior regions of the 6th cervical vertebra were set as the origin “O” of the coordinate, and starting from the origin, the straight line connecting the anterior and inferior regions of the 2nd cervical vertebra was set as the y-axis. With the y-axis passing the origin as the center, the perpendicular line was set as the x-axis ( Fig. 1 ). Based on the Pythagorean Theorem, the distance between variables was calculated with the two points on the coordinate grid using Excel. Using the motion analysis program, a two-dimensional motion analysis of the movement of the hyoid bone, cricoid cartilage, and thyroid cartilage was conducted. Variables measured included the following: the distance from the 6th cervical vertebra to the cricoid cartilage (C6CD, ¯OA
O A ¯
); the distance from the 6th cervical vertebra to the lower part of the thyroid cartilage (C6TD, ¯OB
O B ¯
); the distance from the 6th cervical vertebra to the hyoid bone (C6HD, ¯OC
O C ¯
)); and the distance from the cricoid cartilage to the thyroid cartilage (CTD, ¯AB
AB ¯
). When raised, the pitch was kept constant, so that the angle of the jaw (∠ODE) would not become larger.

Statistical analysis was performed using the Mann–Whitney U test with SPSS 20.0. The pitch was the dependent variable, and a simple linear regression analysis and quadratic regression analysis were carried out with independent variables, C6CD, C6TD, C6HD, and CTD. Adjusted R ² was calculated to see how much the independent variable explains the dependent variable. The explanatory power of the adjusted R ² means the degree of explanation on pitch by each variable of larynx movement.