Physiological Factors of F0 control

A basic function of the larynx and the mechanics of voice production are necessary to be understood before discussing the production of F0 can be discussed. The larynx plays the role of breathing, speaking and swallowing in the human body. The epiglottis is closed to ensure that food will pass through the pharyngeal cavity into the esophagus. In speech, the larynx is important as an articulator and a source of sound.

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Figure 1. Anterior view of larynx

(adopted from http://en.wikipedia.org/wiki/File:Larynx_external_en.svg)

The larynx (Fig. 1) has a skeletal frame formed by a series of cartilages. There are two main cartilages—the upper thyroid cartilage and the lower and smaller cricoids cartilage. The epiglottis lies superiorly; it protects the larynx during swallowing and prevents the inspiration of food.

The most prominent laryngeal cartilage is called the thyroid cartilage. It consists of two plates which are arranged in a wedge-like shape. The hyoid bone is found above the thyroid cartilage. It is connected to the larynx by the thyrohoid membrane. The U-shaped hyoid bone serves as an attachment point for the tongue muscles. Beneath the thyroid cartilage is the cricoids cartilage, which forms the base of the larynx. The anterior part of the cricoids cartilage is narrow and referred to as the arch. The

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posterior part, which is called lamina, is much broader and forms much of the larynx‘s back wall. The cricoids cartilage supports the thyroid cartilage and the arytenoids (see Fig. 2). Its upper edge from four articulatory surfaces: two at the side for the thyroid and two at the back for the arytenoids. Above to the lamina are the arytenoids cartilages, which attach to the vocal folds. A pair of triangle-shaped arytenoids cartilages is located along the upper edge of cricoids lamina. On the top of each arytenoid cartilage is a small corniculate cartilage. Each arytenoid cartilage attaches itself to the posterior end of a vocal ligament (Marchal, 2009).

Figure 2. The larynx seen from the back and right side (adopted from Lai (2009))

Vocal Folds

F0 control is at the larynx is considered to be achieved by the adjusting the effective and the stiffness of the vocal folds (Hirose, 1997). The studies of vocal folds have been investigated for many years (Hirano, 1981; Hirano, 1983), with the objective of

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understanding the behavior of the folds during speech. The vocal folds are twin infoldings of membranes and muscular fibers stretched horizontally across the larynx.

They are located below the epiglottis. They are attached at the back to the processes of the arytenoids cartilage and at the front to the thyroid cartilage. Above the vocal folds is a similar structure known as the false vocal cords. There are no significant contributions of the false vocal cords to the normal vocal folds. The vocal folds and the space between them are referred to as the glottis. The glottis expands into a triangular-shape opening while breathing. This allows oxygen to enter the trachea and lungs. When people hold their breath, for example, the vocal folds are closing. When humans breathe, the vocal folds are opening and vibrating as air passes through the larynx, which also occurs when people speak or sing (known as phonation). To make sound, the laryngeal muscles adduct the size of the opening to a narrow slit.

A first factor influencing the rate of vocal fold vibration is the length of vocal folds.

Long vocal folds oscillate at a slower rate than shorter ones. The length of the vocal folds is 18-24 mm in a man and 14-19 mm in a woman. Since the vocal folds are longer for men than for women, male voices are usually lower than female voices. A second factor determining the rate of vocal fold vibration is the mass, which is also linked to their thickness (Reetz and Jongman, 2009). Thick vocal folds oscillate at a lower rate than thin vocal folds. In addition, the rate of vocal fold vibration depends on the elastic tension: tense folds vibrate faster than slack folds, because they are pulled back to the rest position with more force. These three effects relate the vibration of the muscular vocal folds.

Except for the factors mentioned above, the vocal folds are different at different ages.

Hirano et al. (1989) previously described several structural changes in the vocal folds

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tissues associated with aging. Some of these included: a shortening of the membranous vocal folds in males, a thickening of the vocal folds mucosa and cover in female, and edema development in the superficial layer of the lamina propria in both sexes. Taking both gender and age into consideration, these intrinsic factors cause a different performance in F0.

Laryngeal Mechanisms Cricothyroid Muscle (CT)

The framework of the larynx consists of four different cartilages: the epiglottis, thyroid, cricoids, and arytenoids cartilages. The cricoids and thyroid cartilages are connected by the cricothyroid joint, while the cricoids and arytenoids cartilages are connected by cricoarytenoid joint (Fig. 3). The movement of the cricothyroid joint changes the length of vocal folds. The movements of arytenoids cartilage contribute to the abduction and adduction of the vocal folds (Hirose_1997).

Movement of cricothyroid and cricoarytenoid are controlled by the intrinsic muscles.

Elongation and stretching of the vocal folds are achieved by the contraction of CT. CT is the important intrinsic muscle for the operation of the cricothyroid articulation which affects F0. Continuous contraction of CT produces an increase of F0;

conversely, relaxation of CT lowers F0 (Gay et al. 1972). CT narrows the angle between the cricoid and thyroid cartilages, increasing tension on the vocal folds.

Vocal folds are attached at the back of the arytenoids cartilage and at the front to the thyroid cartilage. Once the CT contracted, thyroid cartilage moves forwards and thus vocal folds are lengthened or stretched by their anterior-posterior plane.

Increased CT activity tenses the vocal folds, which helps to suppress voicing in

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voiceless consonants. This behavior in term leads to a higher F0 on the onset of phonations for the adjacent following vowels (Hoole, 2004; Honda 2004; Whalen et al., 1999). Higher CT activity suggests a raising pattern of F0 (Vilkman, Aatonen, Laine & Raimo, 1989). Researches (Whalen et al., 1999; Vilkman, Aatonen, Laine &

Raimo, 1989) indicate that CT has a higher correlation with the rising and falling of F0, unlike the findings in Honda‘s (1983) research. Honda (1983) found a paradoxical CT activity in the lower F0 region of a speaker‘s range (in this case, at the end of sentence); the increases in CT were correlated with decreases of F0.

Figure 3. The intrinsic muscles of larynx (adopted from Honda (2004))

Other intrinsic muscles

Vocal folds are also affected by other adductor and abductor muscles. The posterior cricoarytenoid muscle (PLA) is the only abductor muscle, while another three—the interarytenoid muscle (INA), lateral cricoarytenoid (LCA) and the thyroarytenoid (TA) muscles—are adductor muscles (Hirose_1997).

Posterior cricoarytenod muscle (PCA) is the biggest and most powerful muscle of the larynx muscles. It extends from the posterior surface of the cricoid to the each side of

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the arytenoid. Contraction of LCA stimulates the translation movement which parts the vocal process. Therefore, the lower vocal folds separated (see Fig. 3b) (Marchal, 2009).

Lateral cricoarytenoid muscle (LCA) originates at the upper edge of the cricoids arch and it insets in the lateral part of the arytenoids cartilage (Fig. 3b). LCA is the smallest of the intrinsic muscles. Its contriction produced the self-pivoting of the arytenoids.

As a result, the vocal processes close and the length of the vibration part of the vocal folds is reduced (Handa, 1983).

Thyroarytenoid muscle (TA) originates on the inner surface of the thyroid cartilage and it is slender at the top and thick at the bottom (Fig. 3b). TA is a very fast muscle.

It opposes the cricoarytenoid muscle and its main function is to draw forward the arytenoids cartilages, thus shortening the vocal folds and decreasing their tension (Marchal, 2009). The cricothyroid (CT) muscle and thyroarytenoid (TA) muscle work together and affect F0. The CT muscles raise F0 by elongating the vocal folds, whereas the TA muscles raise F0 by increasing the stiffness of vibration when F0 is low.

Extra-laryngeal Mechanisms Laryngeal Height

It has been shown by previous researches that the larynx moves up and down as F0 rises and falls (Honda, 1999; Steven, 1977). Vilkman (1996) proposed that the vertical movement of larynx played a role in determining F0. Magnetic resonance image (MRI) recoding of the head and neck region were obtained for three male subjects to tracings of the jaw, hyoid bone, laryngeal cartilage, and cervical spine were compared

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in high and low F0 range. In the high F0 range, the hyoid bones moved horizontally while the larynx height remained relatively constant. In the low F0, the results indicate that vertical movement of the larynx is a crucial component of the F0 control mechanism.

In addition to fixing the larynx to neighboring organs, extrinsic muscles are responsible for the vertical movement of larynx. They also induce changes in the degree of vocal folds. Bothorel (1980) established the incidence of vertical movement by the hyoid bone during speech. He found that the hyoid bone is systematically higher for voiceless consonants than for voiced consonant. Furthermore, He noticed a correlation between the elevation of the hyoid bone and rises in F0. In high F0 range, hyoid bones are responsible for higher F0.

Extrinsic muscles

How does the extrinsic muscle affect the larynx height? Marchel (2009) pointed out that the direction action muscle on the vertical movement of larynx is stylopharyngeus muscle. Stylopharyngeus muscle originates at the base of the styloid and is attached by pharyx, epiglottis, the upper bone of thyroid cartilage, and the upper edge of the cricoids cartilage. Marchal (2009) indicated that stylopharygeus muscle raises the pharynx and the larynx. Once the stylopharyngeus muscle raises the larynx height, F0 increases. Apart from the elevators muscle of the larynx, the sternothyroid is the depressor of larynx (Fig. 4). This muscle runs from the sternum to the thyroid cartilage. In contracting, it fixes the attachment point of the thyroid and lowers the larynx. Consequently a lowering of F0 is observed (Swashima and Hirose, 1983).

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Figure 4. The extrinsic muscles of larynx (adopted from

http://www.rci.rutgers.edu/~uzwiak/AnatPhys/APFallLect14.html)

2.2.2 Aerodynamic factors