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An MRI analysis of the extrinsic tongue muscles during vowel production
Speech Communication 49 (2007) 49–58 www.elsevier.com/locate/specom
An MRI analysis of the extrinsic tongue muscles during vowel production Sayoko Takano *, Kiyoshi Honda
1
ATR Human Information Science Laboratories, BPI, 2-2-2 Hikaridai, Keihanna Science City, Kyoto 619-0288, Japan Received 21 November 2005; received in revised form 31 August 2006; accepted 7 September 2006
Abstract Functions of the extrinsic tongue muscles in vowel production were examined by measurements of muscle length and tongue tissue deformation using MRI (magnetic resonance imaging). Results from the analysis of Japanese vowel data suggested: (1) Contraction and relaxation of the three subdivisions of the genioglossus (GG) play a dominant role in forming tongue shapes for vowels. (2) The extralingual part of the styloglossus (SG), which was previously thought to cause a high-back tongue position by pulling its insertion point in the tongue, was found to be nearly constant across all vowels both in length and orientation. (3) The tongue shape for back vowels is mainly achieved by internal deformation of the tongue tissue, and the medial tissue of the tongue showed lateral expansion in front vowels, and medial compression in back vowels. 2006 Published by Elsevier B.V. Keywords: Tongue muscles; Muscle geometry; Vowel production; MRI
1. Introduction Speech is a human-specific capacity for communication that relies on the faculty of the human vocal organs, and its production is due in large part to the activities of the human tongue to control the acoustic resonance of the vocal tract. Despite their importance in human speech communication, the physiological mechanisms of the tongue muscles are poorly understood, or only assumed by rudimentary information based on gross anatomy and muscle action potentials. This study aims to re-evaluate the previously reported functions of the extrinsic tongue muscles through measurements of muscle length and tongue tissue deformation using magnetic resonance imaging
*
Corresponding author. Present address: Department of Diagnostic Radiology, RWTH University Hospital Aachen, Pauwelsstrasse 30, Aachen 52074, Germany. Tel.: +49 (0)241 80 80537; fax: +49 (0)241 803380537. E-mail addresses: [email protected] (S. Takano), honda@ atr.jp (K. Honda). 1 Tel.: +81 774 95 1051. 0167-6393/$ - see front matter 2006 Published by Elsevier B.V. doi:10.1016/j.specom.2006.09.004
(MRI) data obtained during Japanese vowel production. The functions of the tongue muscles in vowel production have been discussed based mainly on anatomical knowledge (Miyawaki, 1973) and electromyographic (EMG) data (Baer et al., 1988). Anatomical literature describes muscle functions as relying on the geometry of the muscles and the expected effects of their shortening (Gray et al., 1858/ 1989). Such descriptions regard only the muscles’ contraction effects and do not explore the mechanism of the threedimensional deformation of the tongue tissue. Many studies have noted that tongue deformation is three-dimensional, as seen in surface reconstruction (Stone and Lundberg, 1996; Engwall, 2003) and computational modeling (Kakita et al., 1985; Wilhelms-Tricarico, 1995). EMG studies have examined the activities of individual muscles during speech production (Hirose, 1971; Baer et al., 1988), and analyses based on such data have proposed a schematic view of the functional relationship among the extrinsic tongue muscles (Honda, 1996; Maeda and Honda, 1994). However, it is generally difficult to speculate about the three-dimensional nature of tongue deformation mechanisms only from muscle action potentials.
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Nomenclature
Geometrical information regarding the muscle bundles is of primary importance for estimating their functions. Miyawaki (1973) and Takemoto (2001) examined tongue muscle morphology based on cadaver specimens. Miyawaki (1973) sketched tongue muscle structure using tongue tissue slices in the sagittal, coronal, and transverse planes. The muscle fibers of the tongue were manually traced on each slice to depict their locations. Takemoto (2001) performed semi-microscopic analysis of the extrinsic and intrinsic muscles of the tongue and revealed that the tongue muscle tissue can be divided into the inner layer with laminar fiber assembly and the outer layer with the extrinsic muscle bundles. These anatomical studies give us reliable knowledge of the tongue muscles for inferring their functions. EMG data also provide useful information for estimating muscle functions. Kakita et al. (1985) used the EMG data from (Baer et al., 1988) to compute the three-dimensional tongue shape employing a finite element method (FEM) model of the tongue. They further interpreted the function of the styloglossus muscle in back vowels: the extra-lingual and intra-lingual bundles of this muscle together generate a combined force that raises the tongue dorsum. Maeda and Honda (1994), also using the same EMG data, proposed an antagonistic scheme among the tongue muscles: vowels are controlled by two orthogonally arranged pairs of antagonistic muscles. The previous studies noted above lack sufficient evidence to determine the functions of the extrinsic muscles, since anatomical descriptions alone offer little insight into threedimensional deformation mechanisms, and EMG data alone fail to explain the quantitative contributions of individual muscles to vowel production. Recently, a few MRI methods were introduced to visualize tongue tissue deformation, such as ‘‘cine-MRI’’ (Masaki et al., 1999) and ‘‘tagging cine-MRI’’ (Stone et al., 2001). In the latter, a mesh pattern is labeled onto the tissue image by adding ‘‘tag’’ pulses to decrease MR signals along parallel lines or a grid, and the deformation of the mesh pattern during a short speech sequence is monitored to infer the changes of tongue muscle geometry. Although this method provides a possible means to investigate tongue muscle functions, image resolution is yet insufficient to identify the precise geometry of each muscle. The present study employs high-resolution static MRI techniques to investigate the functions of the extrinsic tongue muscles in vowel production. Image analyses for muscle shortening and tongue deformation were carried out on the basis of the following assumptions: (1) The static
MRI EMG / /
magnetic resonance imaging electromyography Japanese mid-high vowel
muscle length reflects an equilibrium state of tongue muscle forces and their effects on tongue deformation. (2) The internal deformation of the tongue tissue helps interpret the contractile effects of deep tongue muscles. (3) The function of an individual muscle can be inferred by exploring the three-dimensional muscle geometry for each vowel. 2. Functions of the extrinsic tongue muscles in the literature This study focuses on the geometry and function of the three extrinsic tongue muscles. They have been described in the literature as follows. The genioglossus (GG) arises from the mental spine of the mandibular symphysis and spreads mid-sagittally in a fan shape in the medial part of the tongue. The posterior fibers (GGp) course to the tongue root, and the anterior fibers (GGa) project toward the dorsal surface of the tongue. The fibers of the GGp are thicker, stronger, and wider at the base of the tongue than the fibers of the GGa. EMG signals show that the GGa and GGp fibers function differently (Hirose, 1971; Baer et al., 1988). The GGp is more active in high vowels and the GGa is more active in front vowels. The hyoglossus (HG) is a thin quadrangular muscle that arises from the bilateral greater horns of the hyoid bone and runs upward and forward along the lateral side of the tongue. EMG data show that the HG is active for low-back vowels (Baer et al., 1988). The styloglossus (SG) is a long bundle of muscle that arises from the styloid process of the temporal bone, runs forward and downward toward the back of the tongue on each side, and ends at the tongue tip. The SG is active during back vowels. In EMG data, the SG is active for back vowels and most extreme for a high-back vowel /u/. 3. Methods 3.1. Muscle length measurements The lengths of the tongue muscles in mid-sagittal projection were measured on sagittal MRI data on the basis of distance scaling of makers and landmarks, and their variation across vowels was analyzed. 3.1.1. Data collection Sagittal MRI data were collected from four male Japanese subjects during sustained production of the five Japanese vowels /i/, /e/, /a/, /o/, and / /. During the m
HG SG
genioglossus (GGa: anterior, GGm: middle, GGp: posterior) hyoglossus (HGa: anterior, HGp: posterior) styloglossus (SGa: anterior, SGm: middle, SGp: posterior)
m
GG
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MRI scan, the subjects took a supine position, and produced a sustained vowel. Four small spherical capsules of 3 mm in diameter were used as MRI markers. Two markers were used to divide the GG into functional divisions: they were attached to the tongue blade (2 cm back from the tongue tip) and the tongue dorsum (the highest point of the tongue visually recognized with the mouth wide open) along the mid-line. Two others were attached to the tip and nasion of the external nose. Volumetric MRI data were obtained with a clinical MRI scanner (Magnex Eclipse, 1.5 T, Shimadzu-Marconi) with a Fast Spin Echo sequence (4-echo protocol, TE = 15 ms, TR = 2764 ms, NEX = 1). Thirty-five sagittal slices were acquired for each vowel (slice thickness = 2.5 mm, slice gap = 0 mm) with the acquisition time of 3 min. In each slice, the field of view (FOV) was 256 · 256 mm, which was sampled as an image of 512 · 512 pixels. Therefore, the image resolution for each volume is 0.5 · 0.5 · 2.5 mm per pixel. 3.1.2. Procedure for muscle length measurements The intrinsic tongue muscles, extrinsic tongue muscles, and oral-floor muscles such as the geniohyoid and mylohyoid were identified in each slice, and their geometrical patterns were traced manually. All the tracings of the muscles were projected onto the sagittal plane, where muscle length measurements were performed. Therefore, muscle lengths measured in this study represent sagittal projections of their true lengths. The lengths of subdivisions of the genioglossus (GG) were measured directly from the mid-sagittal images because the GG is situated mid-sagittally. The two other extrinsic muscles, the hyoglossus (HG) and styloglossus (SG) were measured from the para-sagittal images. This report describes the results for the three extrinsic tongue muscles. The GG is anatomically arranged as a triangular muscle, and it is reasonable to assume that it has three functional subdivisions, i.e., anterior (GGa), middle (GGm), and posterior (GGp). Each subdivision was defined in the mid-sagittal slice, as shown in Fig. 1. The entire GG in the mid-sagittal plane was bounded by two lines: a line from the mental spine to the tongue blade marker and a line from the mental spine to the upper ridge of the body of the hyoid bone. This region was divided into three subdivisions by two boundary lines, as shown in Fig. 1. The GGa–GGm boundary is the line from the mental spine to the tongue dorsum marker, and the GGm–GGp boundary is the line that extends along the orientation of the short tendon of the GG, which is seen in the image of the GG as a bright line arising from the mental spine of the mandibular symphysis. The length of each subdivision was defined as the length of the line segment bisecting each fan-shaped subdivision from the mental spine to the tongue surface shown by thin white lines with black edges. These bisecting lines were used for measurement because they are geometrically suitable to represent each of the GG’s subdivisions and are less sensitive to the possible subjectto-subject variation in marker placement.
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Fig. 1. Method for length measurement of the extrinsic tongue muscles. ‘‘GG’’, ‘‘SG’’, and ‘‘HG’’ stand for the genioglossus, styloglossus, and hyoglossus muscles, respectively. The letters ‘‘a’’, ‘‘m’’, and ‘‘p’’ mean anterior, middle, and posterior, respectively. A is the turning point of the SG and B is the point of intersection of the SGp and HGa.
The HG is a flat rectangular muscle that arises from the hyoid bone, and merges with the intra-lingual part of the SG at two landmark points (A and B), as shown in Fig. 1. The HG was measured at the anterior (HGa) and posterior (HGp) edges. The length of the HGa is the line segment from the anterior edge of the body of the hyoid bone to Point B, and the length of the HGp is the line segment from the posterior end of the greater horn of the hyoid bone to Point A. The SG is a long thin muscle that arises from the styloid process of the temporal bone, and runs forward and downward to enter the side of the tongue near the mid-pharynx wall to extend toward the tongue tip. In this measurement, the SG was classified into three segments: SGp, SGm, and SGa according to the muscle’s geometrical pattern. The SG posterior (SGp) is the length of the extra-lingual bundle from the styloid process to Point A. The SG middle (SGm) is the length from Point A to Point B, and the SG anterior (SGa) is the length from Point B to the tongue tip, both corresponding to the intra-lingual bundles. 3.2. Measurements of three-dimensional tongue shape Three procedures were employed to describe the threedimensional shapes of the tongue: two were for the measurements of the lateral deformation (width between bilateral arterial landmarks, and sagittal cross-sectional area of the tongue), and the other was for a comparison of the shape of the tongue surface (extraction of the contours of reconstructed three-dimensional surface models). 3.2.1. Data collection One male speaker among the four initial MRI subjects served as a subject for obtaining both coronal and sagittal
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slices of the tongue during production of the five vowels. MRI data of a smaller slice thickness of 2.0 mm with no inter-slice gap were collected by using the same MRI scanner with a Fast Spin Echo (4-echo protocol, TE = 15 ms, TR = 3680 ms, NEX = 1, and FOV 256 · 256 mm). For each vowel, 51 sagittal slices and 51 coronal slices were recorded. The acquisition time for each volume was about 4 min. The FOV of 256 · 256 mm was sampled as 512 · 512 pixels, with the image resolution of 0.5 · 0.5 · 2.0 mm per pixel. 3.2.2. Measurements of the distance between bilateral arterial landmarks A sharp bending point of the deep branch of the lingual arteries was identified on each side of the tongue by using the sagittal and coronal images, and the distance between the right and left landmark points was measured to index the lateral deformation of the mid-line tongue tissue (Fig. 4).
3.2.4. Comparison of three-dimensionally reconstructed shapes of the tongue Three-dimensional shapes of the tongue during production of five Japanese vowels were reconstructed as computer graphics models based on the sagittal and coronal MRI slices. The bottom boundary of the tongue model was the bottom of the GGp as described above. The lateral boundaries of the tongue were determined by the lateral surface of the HG for the posterior part of the tongue and by the oral surface of the mandible for the anterior part. 4. Results The MRI data collected for the measurements of muscle lengths and tongue shape changes were found to have sufficient quality for image analysis. All of the four subjects produced the mid-vowel / / regardless of the subjects’ dialects. m
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4.1. Muscle lengths Fig. 2 gives examples of the data from one subject to show the geometries of the extrinsic tongue muscles and bar graphs of muscle lengths for the GGa, GGp, HGa, and SG (three segments) in the five vowels. The mid-sagittal outlines of the tongue for the five vowels show two characteristic patterns of Japanese vowel production. First, while the tongue dorsum is higher in high vowels (/i/ and / /) and lower in a low vowel (/a/), the highest point of the tongue dorsum for / / is not in the rearmost but is between /e/ and /o/. Second, the tongue tip tends to remain m
m
3.2.3. Measurements of the sagittal cross-sectional areas of the tongue Due to volume conservation of the tongue, the lateral deformation of the tongue tissue is also reflected in the change in the mid-sagittal cross-sectional area of the tongue. The outlines of the tongue were manually traced on 11 sagittal slices, and tongue tissue area was measured on each slice. In this measurement, the bottom of the tongue was defined by the bottom margin of the GGp, i.e., a line from the mental spine to the upper ridge of the body of the hyoid bone (see Fig. 1).
Fig. 2. Example of the muscle lengths for one subject. The GGp is shorter in the front vowels /i/, /e/, and, / /. The GGa and GGm are short in the back vowels /a/ and /o/. The HGa is short in the back vowels /a/ and /o/. According to the geometry and length of the tongue muscles, Japanese vowels can be roughly divided into two clusters, a high-front type (/i/, /e/, and / /) and a low-back type (/a/ and /o/). m
m
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similar patterns: The GGm and GGp are longer in /a/ and /o/ (convex lines in the graph), and the GGa, SGm, and HGa are longer in /i/, /e/, and / / (concave lines). This patterning is largely due to the characteristics of the articulation of Japanese vowels having two clusters of front–mid-vowels (/i, , e/) and back vowels (/a, o/). m
m
4.2. Distances between arterial landmarks Fig. 4 shows the locations of the right and left arterial landmarks and their linear distance. The landmarks were defined by an acute curvature of the deep lingual arteries that run between the GG and HG. These curvature points were observed in the lateral aspect of the posterior part of the GGm in this subject. These landmarks move with the tongue tissue, displacing forward in front vowels and backward in back vowels. As shown by the bar graph in Fig. 4, the distance between the two arterial landmarks was smaller in /a/ and /o/, and larger in /i/, /e/, and / /. This result indicates that the mid-line tongue tissue between the bilateral landmarks expands laterally in /i/, /e/, and / /. m
m
4.3. Sagittal cross-sectional areas of the tongue The sagittal cross-sectional areas of the tongue were measured to examine the three-dimensional tongue deformation in vowels. Fig. 5 shows the areas of 11 sagittal sections of the tongue measured for a 2-cm thick segment. In the figure, the areas near the mid-sagittal sections are smaller in /i/, /e/, and / / than those of the lateral sections. In contrast, the areas are nearly constant in all the sections in /a/ and /o/. The volume of the tongue for the measured m
in front even in low-back vowels (/a/ and /o/) and the tongue blade shows a downward indentation, which is called the ‘‘lingual fossa.’’ These characteristics are commonly found in all the subjects. According to the bar graphs, the GGa and GGp show a reciprocal pattern: the GGp is shorter in /i/ and / /, while the GGa is shorter in /a/ and /o/. The HGa and SG tend to be shorter in low-back vowels than in other vowels. The HGa is the shortest in /a/, while the SG is the shortest in /o/. Table 1 lists the measured lengths of the extrinsic tongue muscles for the four subjects for the five vowels and their changes in percentage with reference to the maximum length of each muscle in the five vowels. The GGp and GGm were shorter in /i/ and / /, and the GGa was shorter in /a/ and /o/. The HGa was shorter in /a/ and /o/, while the HGp showed an inconsistent tendency among the subjects. The total length of the SG was observed to be relatively constant compared to the obvious changes in length of the other muscles. The SGa showed individual differences reflecting the positional variation of the tongue tip across subjects. In subject S1, the SGa is longer in /a/ and /o/ than in /i/, /e/ and / /. This is because the tongue tip of this subject remained in front even for back vowels. The SGm was shorter in /a/ and /o/ than in /i/, /e/, and / / for all the subjects. The SGp remained nearly the same both in length and orientation across vowels in all the subjects. Fig. 3 shows the subject-to-subject variation among the measured lengths by plotting the deviation from the mean for each subject according to the data in Table 1. In Fig. 3, the GGp and GGa show the largest changes in length across vowels, while the length of the SGp shows the smallest changes in all the subjects. Some of the muscles show
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m
m
m
m
Table 1 The changes of the muscle length for each vowel in the four subjects GGa
GGm
GGp
SGp
SGm
SGa
HGa
HGp
GGa
GGm
GGp
SGp
SGm
SGa
HGa
HGp
S1
/i/ /e/ /a/ /o/ / / m
47 44 34 33 47
49 51 57 58 55
49 49 51 54 50
51 55 49 47 52
28 28 20 22 23
42 39 50 49 38
38 39 34 30 35
30 28 34 32 31
100.0 93.6 72.3 70.2 100.0
84.5 87.9 98.3 100.0 94.8
90.7 90.7 94.4 100.0 92.6
92.7 100.0 89.1 85.5 94.5
100.0 100.0 71.4 78.6 82.1
84.0 78.0 100.0 98.0 76.0
97.4 100.0 87.2 76.9 89.7
88.2 82.4 100.0 94.1 91.2
S2
/i/ /e/ /a/ /o/ / / m
51 48 41 41 46
56 56 62 66 60
40 47 53 50 45
60 60 59 57 59
28 29 23 22 32
42 42 44 43 42
40 38 33 37 42
31 29 35 35 31
100.0 94.1 80.4 80.4 90.2
84.8 84.8 93.9 100.0 90.9
75.5 88.7 100.0 94.3 84.9
100.0 100.0 98.3 95.0 98.3
87.5 90.6 71.9 68.8 100.0
95.5 95.5 100.0 97.7 95.5
95.2 90.5 78.6 88.1 100.0
88.6 82.9 100.0 100.0 88.6
S3
/i/ /e/ /a/ /o/ / / m
50 44 46 46 50
53 55 58 59 59
43 50 58 58 48
43 42 39 39 42
28 27 21 22 25
36 37 35 32 35
35 32 27 27 33
28 28 33 35 30
100.0 88.0 92.0 92.0 100.0
89.8 93.2 98.3 100.0 100.0
74.1 86.2 100.0 100.0 82.8
100.0 97.7 90.7 90.7 97.7
100.0 96.4 75.0 78.6 89.3
97.3 100.0 94.6 86.5 94.6
100.0 91.4 77.1 77.1 94.3
80.0 80.0 94.3 100.0 85.7
S4
/i/ /e/ /a/ /o/ / /
47 48 34 35 45
59 56 62 66 59
45 53 62 59 47
46 47 47 46 46
32 32 28 28 32
43 43 47 45 43
28 25 22 23 28
31 28 22 25 27
97.9 100.0 70.8 72.9 93.8
89.4 84.8 93.9 100.0 89.4
72.6 85.5 100.0 95.2 75.8
97.9 100.0 100.0 97.9 97.9
100.0 100.0 87.5 87.5 100.0
91.5 91.5 100.0 95.7 91.5
100.0 89.3 78.6 82.1 100.0
100.0 90.3 71.0 80.6 87.1
m
Left: length in mm, right: length in percent relative to the maximum length of each muscle.
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15 10 5 0 /i/
/e/
/a/
/o/
/m/
Landmark distance [mm]
Fig. 3. A graph of the muscle length deviation from the mean for the five vowels.
Fig. 4. The distance between the right and left landmark points. Each landmark is the sharp curve of the deep branch of the lingual artery.
segment (the area under the curve in the figure) is larger in /a/ and /o/, and smaller in /i/, /e/, and / /. The contrast of the Japanese vowel clusters is found in the area measurement. The result of the area measurement among the eleven 2cm thick sagittal slices of the tongue indicates that vowel articulation involves three-dimensional tongue deformation. The smaller mid-sagittal areas in /i/, /e/, and / / indicate that the mid-line tongue tissue is sagittally compressed to expand laterally. The tissue pushed aside from the midline possibly contributes to bulging the tongue tissue on both sides of the mid-line groove in these vowels. In contrast, the larger mid-sagittal areas in /a/ and /o/ explain that the tongue tissue is compressed medially toward the mid-line to expand sagittally. The tongue tissue gathered
in from the sides contributes to forming a bunched shape of the tongue dorsum in /a/ and /o/. 4.4. Three-dimensional reconstruction of the tongue surface Fig. 6a shows the front views of the tongue models for the three vowels /i/, /a/, and /o/. The geometry of the extra-lingual part of the SG is nearly constant across vowels despite the obvious changes in the whole tongue shape. The asymmetry of the tongue shape is extreme in /i/ but less obvious in /a/ and /o/. The asymmetry of the tongue in /i/ reflects the asymmetrical shape of the palate and causes the off-mid-line deviation of the front part of the vocal tract. In evidence, the MRI data show that the right roof of the hard palate is higher than the left roof in this
m
m
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Fig. 5. Result of tongue area measurement in the mid-sagittal plane and five para-sagittal planes for both sides. The 11 slices correspond to the tissue segment of 2-cm thickness. The right-top shows the mid-sagittal plane of MRI with the measured area of the tongue. The right-bottom indicates the measured portions that were seen from the front.
Fig. 6. (a) The front view of the three-dimensional tongue model in /i/, /a/, and /o/. The geometry of the SG is almost constant, while the insertion point of the SGp moves slightly possibly due to jaw movement. (b) The top view of the three-dimensional tongue model in /i/, /a/, and /o/. As the tongue moves forward and backward, the shape of the pharyngeal surface of the tongue changes, from concavity in front vowels to convexity in back vowels, whereas the SG maintains the same geometry. The tongue blade markers disappear in the 3D reconstructions of the tongue for /i/ and /a/ due to the downward chemical shift of the markers on the original images and surface rendering for the reconstructions.
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subject. The lateral width of the tongue varied minimally, even though considerable deformation of the internal tongue tissue was observed. Fig. 6b shows the top views of the vowels /i/, /a/, and /o/. The pharyngeal surface of the tongue is posterior in /a/ and /o/, and front in /i/. Nonetheless, the geometry of the SG does not vary much and the location of the insertion point of the SG to the tongue is nearly constant in all the vowels. Also, the tongue width does not differ greatly regardless of the position of the tongue body. The pharyngeal surface of the tongue is convex forward in /i/, whereas it is convex backward in /o/. These observations suggest that the back positions of the tongue for back vowels are caused principally by internally accumulated strain rather than by external traction force. 5. Discussion In this study, the lengths of the extrinsic tongue muscles were measured and the three-dimensional deformation of the tongue tissue was visualized for the five Japanese vowels. In this section, the results obtained are compared with previous descriptions in the literature, and the roles of the extrinsic tongue muscles in vowel articulation are discussed. 5.1. Functions of the extrinsic tongue muscles 5.1.1. The genioglossus (GG) The GG is the largest and strongest muscle in the tongue (Abd-el-malek, 1939; Gray et al., 1858/1989; Takemoto, 2001). Due to its triangular configuration, this muscle can generate basic patterns of tongue deformation for vowels by a combination of contraction and relaxation among the subdivisions. The vowel-to-vowel changes in muscle length of the three subdivisions of the GG were the most extensive among the extrinsic tongue muscles, as shown in Fig. 3. These observations suggest that the GG plays a dominant role in determining the tongue shape in vowel articulation. The GGa is defined in this study as a subdivision of the GG that runs vertically. The GGa was shorter in back vowels than in front vowels. This suggests its contribution to tongue backing and bunching in /a/ and /o/. Supposing that the tongue tissue behaves as a hydrostat, the shortening of the GGa should result in a backward bulging of the tongue body, when the GGm and GGp are inactive. The tongue blade in the mid-sagittal MRI slice often shows the concavity of the lingual fossa, which also supports the role of the GGa in back vowel articulation. These characteristics are typical for Japanese back vowels. Considering the sparse arrangement of muscle bundles in this region, however, this effect alone may not be sufficient to complete the shape of the tongue for back vowels. The GGm is an oblique muscle bundle in the tongue, and it was shorter in front vowels than in back vowels in the result of our muscle length measurement (see Table 1 and
Fig. 3). The contraction of this bundle alone can compress the mid-sagittal tongue tissue to form an oval cross-section, driving the free tongue blade forward and the root downward. Since the downward force is limited by the mass beneath (i.e., the GGp), contraction of the GGm is thought to advance and elevate the front part of the tongue. The GGm in this study can be estimated to correspond to the ‘‘GGa’’ in the EMG study by Baer et al. (1988), because they divided the GG into two portions (GGa and GGp). The GGp is the largest subdivision among the GG because it spreads laterally toward the tongue root (Abd-el-malek, 1939). This muscle is known to be most active in high vowels /i/ and /u/, and produced the largest signals in the EMG recordings of Baer et al. (1988). The contraction of this bundle advances the tongue root, elevating the tongue tissue above, and thus forms the tongue shape of high vowels. In the MRI data, the length of the GGp changes dramatically across vowels, which suggests that the GGp is the muscle that most strongly determines vowel articulations. It is reasonable to speculate that contraction of the GG causes lateral expansion of the internal tissue of the tongue, as seen in the variation of the distance between arterial landmarks. The deep lingual artery runs along the lateral margin of the GGp and GGm, and the contraction of these muscles should produce a change in the width of the muscle bundles. As evidence, in the front vowels /i/ and /e/, the area of the mid-sagittal cross-section was smaller than the area of the para-sagittal sections. This implies that the GGp and GGm together produce a high-front position of the tongue body while expelling the mid-line tongue tissue to the sides. 5.1.2. The hyoglossus (HG) The HG is a thin quadrangular muscle, which runs along the lateral backside of the tongue, and blends with muscles such as the SGm in the sides of the tongue. The length of the HG was measured in two portions, i.e., anterior (HGa) and posterior (HGp). The length of the HGa was shorter in back vowels than in front vowels, which agrees with the EMG data (Baer et al., 1988) as well as with the general assumption that the activation of the muscle causes a low-back position of the tongue body. The tongue model simulation by Perkell (1996) reports that the HGp was more dominant than the HGa for producing tongue dorsum bulging for the English back vowel /a/. The disagreement between Perkell’s and the present studies may be due to language-specific differences in vowel articulation: the tongue dorsum for /a/ is lower in English while it is higher for /a/ in Japanese. 5.1.3. The styloglossus (SG) In this study, the SG was defined as three segments: two intra-lingual (SGa and SGm) segments and one extra-lingual (SGp) segment. It has been believed that the SGp draws the tongue body obliquely toward the direction of the styloid process (Baer et al., 1988), being antagonistic
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to the oblique bundle of the GG (GGm) (Maeda and Honda, 1994; Honda, 1996). While this assumption is natural from a lateral view of this muscle in anatomical literature, it differs from the result of our measurements due to the following three reasons. First, the length and orientation of the SGp are nearly unchanged across vowels as seen in Figs. 2 and 3. This observation may in part be due to the fact that the SGp is surrounded by soft tissue (Gray et al., 1858/1989), which allows muscle shortening with no obvious displacement of the muscle in the direction perpendicular to its axis of contraction. Even in the case that the SGp slides in the conduit formed by a soft tissue wall as it contracts, the direction of the force to pull the tongue might not be toward the styloid process, but simply backward. Second, the insertion point of the SG on the tongue body is too low to form the extreme high-back position of the tongue as seen in the English /u/ by directly pulling the muscle’s insertion point. Third, the shape of the pharyngeal surface of the tongue viewed from the top is convex backward in /o/ and /a/ (see Fig. 6a), implying an effect of intra-lingual deformation to form a bunched shape of the tongue. Perkell (1996) also notes the function of the intra-lingual segment of the SG to produce upward bulging of the tongue dorsum. All of these observations suggest that a back vowel articulation is not primarily caused by traction forces of the SGp and HG but by tissue stress generated by several muscles, such as the SGm, GGa and HGa. In addition, the contraction of the transverse muscle and the relaxation of the GGp may also be involved in the production of low-back vowels. 5.2. Vowel production mechanisms Physiological mechanisms for producing Japanese vowels are proposed below based on the results of the MRI analysis regarding the geometries of the extrinsic tongue muscles. 5.2.1. High-front vowels The GG is the dominant muscle in determining the tongue shape for vowels. When the entire GG is activated, the GGp powerfully draws the tongue root forward, and advances the tongue body. The contraction of the GG also results in the lateral expansion of the mid-line tongue tissue and the compression of the lateral tissue. Since the lateral aspect of the tongue is limited by the mandible, the whole tongue is raised toward the palate due to the internal strain mainly effected by the GGp. The GGp has a major effect on the entire tongue, because it has the largest volume and can apply the greatest force. The GGa can be lengthened passively even if it is active (Baer et al., 1988). The GGa maintains the mid-line groove of the tongue and maintains a small front cavity for front vowels. 5.2.2. Low-back vowels In back vowels, the GGp and GGm are relaxed to allow the tongue shape and position to be controlled by other
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muscles. The SGm contracts for back vowels and helps form the tongue into a bunched shape. In low-back vowels, the HGa contracts to lower the tongue and make it bulge out in a position and direction. The GGa contributes to lowering the tongue blade or making the anterior mid-line groove, which also helps keep the unique shape of the tongue blade with the lingual fossa in Japanese vowels. Contrary to previous accounts, the SGp alone cannot draw the tongue body in the direction of the styloid process, but the SGm contributes to forming the bunched shape of the tongue body for back vowels.
6. Conclusion The tongue muscles make the most significant contributions to the articulation of speech sound. Despite their critical functions supporting human communication, knowledge of how each muscle performs its role has mostly been speculative. While EMG and anatomical data have been the major source of information for estimating the physiological function of the tongue muscles, these data do not provide direct evidence of the causal relationships. This is because the tongue muscles blend with each other and interact mutually in a three-dimensional manner. Visualization of muscle bundles during static speech gestures, as described in this study, should supplement our understanding of the functions of the tongue muscles and the mechanisms of speech production, even though measurements of muscle geometry do not directly index muscle activity but reflect an equilibrium state among the muscles. The results from this study that aimed at revealing the physiological mechanisms of vowel production, are summarized as follows: (1) The entire genioglossus (GG) demonstrated the greatest change in length across vowels, suggesting that it plays a dominant role in producing front vowels by contraction and assists back vowels by relaxation. In comparison, the styloglossus (SG) and the hyoglossus (HG) show a smaller extent of length changes, inferring a less deterministic role in vowel production. (2) The SG was previously thought to pull the tongue in the direction of the styloid process, which was not supported by the present study because the extra-lingual part of the muscle (SGp) remained constant in length and orientation. Instead, the intra-lingual part of the muscle (SGm) appears to be the most critical for the bunching of the tongue in a backward direction. (3) The tongue shape for vowel production is caused by the internal deformation of the tongue tissue. In front vowels, the internal tissue of the tongue is expanded laterally. In back vowels, the tongue is compressed medially.
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Acknowledgements We are grateful to Dr. Shinobu Masaki, Mr. Yasuhiro Shimada, and Mr. Ichiro Fujimoto at the ATR Brain Activity Imaging Center for their help in conducting this study. We also thank Dr. Masaaki Honda at Waseda University for his effort in realizing the work as the P.I. of the CREST project. This work was conducted while the first author belonged to the CREST of JST (Japan Science and Technology). This research was also supported in part by the National Institute of Information and Communications Technology. References Abd-el-malek, S., 1939. Observations on the morphology of the human tongue. J. Anat. 73, 201–210. Baer, T., Alfonso, P.J., Honda, K., 1988. Electromyography of the tongue muscles during vowels in /EpVp/ environment. Ann. Bull. RILP 22, 7– 19. Engwall, O., 2003. Combining MRI, EMA and EPG measurements in three-dimensional tongue model. Speech Commun. 41 (2–3), 303–329. Gray, H., Williams, P.L., et al. (Eds.), 1989. Gray’s Anatomy, 37th ed. Churchill Livingstone, Edinburgh (Original work published 1858). Hirose, H., 1971. Electromyography of the articulatory muscles: current instrumentation and technique. Haskins Laboratories Status Report SR-25/26, pp. 73–86.
Honda, K., 1996. Organization of tongue articulation for vowels. J. Phonetics 24, 39–52. Kakita, Y., Fujimura, O., Honda, K., 1985. Computation of mapping from muscular contraction patterns to formant patterns in vowel space. In: Fromkin, V.A. (Ed.), Phonetic Linguistic. Academic Press, New York, pp. 133–144. Maeda, S., Honda, K., 1994. From EMG to formant patterns of vowels: the implication of vowel spaces. Phonetica 51, 17–29. Masaki, S., Tiede, M., Honda, K., 1999. MRI-based speech production study using a synchronized sampling method. J. Acoust. Soc. Jpn. (E) 20, 375–379. Miyawaki, K., 1973. A study on the musculature of the human tongue – observations on the transparent preparations of serial sections. Ann. Bull. RILP 8, 23–49. Perkell, J.S., 1996. Properties of the tongue help to define vowel categories: hypotheses based on physiologically-oriented modeling. J. Phonetics 24, 3–22. Stone, M., Lundberg, A., 1996. Three-dimensional tongue surface shapes of English consonants and vowels. J. Acoust. Soc. Amer. 99, 3728– 3737. Stone, M., Davis, E., Douglas, A., 2001. Modeling the motion of the internal tongue from tagged cine-MRI images. J. Acoust. Soc. Amer. 109, 2947–2982. Takemoto, H., 2001. Morphological analyses of the human tongue musculature for three-dimensional modeling. J. Speech Lang. Hear. Res. 44, 95–107. Wilhelms-Tricarico, R., 1995. Physiological modeling of speech production: methods for modeling soft-tissue articulators. J. Acoust. Soc. Amer. 97, 3085–3098.
An MRI analysis of the extrinsic tongue muscles during vowel production Sayoko Takano *, Kiyoshi Honda
1
ATR Human Information Science Laboratories, BPI, 2-2-2 Hikaridai, Keihanna Science City, Kyoto 619-0288, Japan Received 21 November 2005; received in revised form 31 August 2006; accepted 7 September 2006
Abstract Functions of the extrinsic tongue muscles in vowel production were examined by measurements of muscle length and tongue tissue deformation using MRI (magnetic resonance imaging). Results from the analysis of Japanese vowel data suggested: (1) Contraction and relaxation of the three subdivisions of the genioglossus (GG) play a dominant role in forming tongue shapes for vowels. (2) The extralingual part of the styloglossus (SG), which was previously thought to cause a high-back tongue position by pulling its insertion point in the tongue, was found to be nearly constant across all vowels both in length and orientation. (3) The tongue shape for back vowels is mainly achieved by internal deformation of the tongue tissue, and the medial tissue of the tongue showed lateral expansion in front vowels, and medial compression in back vowels. 2006 Published by Elsevier B.V. Keywords: Tongue muscles; Muscle geometry; Vowel production; MRI
1. Introduction Speech is a human-specific capacity for communication that relies on the faculty of the human vocal organs, and its production is due in large part to the activities of the human tongue to control the acoustic resonance of the vocal tract. Despite their importance in human speech communication, the physiological mechanisms of the tongue muscles are poorly understood, or only assumed by rudimentary information based on gross anatomy and muscle action potentials. This study aims to re-evaluate the previously reported functions of the extrinsic tongue muscles through measurements of muscle length and tongue tissue deformation using magnetic resonance imaging
*
Corresponding author. Present address: Department of Diagnostic Radiology, RWTH University Hospital Aachen, Pauwelsstrasse 30, Aachen 52074, Germany. Tel.: +49 (0)241 80 80537; fax: +49 (0)241 803380537. E-mail addresses: [email protected] (S. Takano), honda@ atr.jp (K. Honda). 1 Tel.: +81 774 95 1051. 0167-6393/$ - see front matter 2006 Published by Elsevier B.V. doi:10.1016/j.specom.2006.09.004
(MRI) data obtained during Japanese vowel production. The functions of the tongue muscles in vowel production have been discussed based mainly on anatomical knowledge (Miyawaki, 1973) and electromyographic (EMG) data (Baer et al., 1988). Anatomical literature describes muscle functions as relying on the geometry of the muscles and the expected effects of their shortening (Gray et al., 1858/ 1989). Such descriptions regard only the muscles’ contraction effects and do not explore the mechanism of the threedimensional deformation of the tongue tissue. Many studies have noted that tongue deformation is three-dimensional, as seen in surface reconstruction (Stone and Lundberg, 1996; Engwall, 2003) and computational modeling (Kakita et al., 1985; Wilhelms-Tricarico, 1995). EMG studies have examined the activities of individual muscles during speech production (Hirose, 1971; Baer et al., 1988), and analyses based on such data have proposed a schematic view of the functional relationship among the extrinsic tongue muscles (Honda, 1996; Maeda and Honda, 1994). However, it is generally difficult to speculate about the three-dimensional nature of tongue deformation mechanisms only from muscle action potentials.
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Nomenclature
Geometrical information regarding the muscle bundles is of primary importance for estimating their functions. Miyawaki (1973) and Takemoto (2001) examined tongue muscle morphology based on cadaver specimens. Miyawaki (1973) sketched tongue muscle structure using tongue tissue slices in the sagittal, coronal, and transverse planes. The muscle fibers of the tongue were manually traced on each slice to depict their locations. Takemoto (2001) performed semi-microscopic analysis of the extrinsic and intrinsic muscles of the tongue and revealed that the tongue muscle tissue can be divided into the inner layer with laminar fiber assembly and the outer layer with the extrinsic muscle bundles. These anatomical studies give us reliable knowledge of the tongue muscles for inferring their functions. EMG data also provide useful information for estimating muscle functions. Kakita et al. (1985) used the EMG data from (Baer et al., 1988) to compute the three-dimensional tongue shape employing a finite element method (FEM) model of the tongue. They further interpreted the function of the styloglossus muscle in back vowels: the extra-lingual and intra-lingual bundles of this muscle together generate a combined force that raises the tongue dorsum. Maeda and Honda (1994), also using the same EMG data, proposed an antagonistic scheme among the tongue muscles: vowels are controlled by two orthogonally arranged pairs of antagonistic muscles. The previous studies noted above lack sufficient evidence to determine the functions of the extrinsic muscles, since anatomical descriptions alone offer little insight into threedimensional deformation mechanisms, and EMG data alone fail to explain the quantitative contributions of individual muscles to vowel production. Recently, a few MRI methods were introduced to visualize tongue tissue deformation, such as ‘‘cine-MRI’’ (Masaki et al., 1999) and ‘‘tagging cine-MRI’’ (Stone et al., 2001). In the latter, a mesh pattern is labeled onto the tissue image by adding ‘‘tag’’ pulses to decrease MR signals along parallel lines or a grid, and the deformation of the mesh pattern during a short speech sequence is monitored to infer the changes of tongue muscle geometry. Although this method provides a possible means to investigate tongue muscle functions, image resolution is yet insufficient to identify the precise geometry of each muscle. The present study employs high-resolution static MRI techniques to investigate the functions of the extrinsic tongue muscles in vowel production. Image analyses for muscle shortening and tongue deformation were carried out on the basis of the following assumptions: (1) The static
MRI EMG / /
magnetic resonance imaging electromyography Japanese mid-high vowel
muscle length reflects an equilibrium state of tongue muscle forces and their effects on tongue deformation. (2) The internal deformation of the tongue tissue helps interpret the contractile effects of deep tongue muscles. (3) The function of an individual muscle can be inferred by exploring the three-dimensional muscle geometry for each vowel. 2. Functions of the extrinsic tongue muscles in the literature This study focuses on the geometry and function of the three extrinsic tongue muscles. They have been described in the literature as follows. The genioglossus (GG) arises from the mental spine of the mandibular symphysis and spreads mid-sagittally in a fan shape in the medial part of the tongue. The posterior fibers (GGp) course to the tongue root, and the anterior fibers (GGa) project toward the dorsal surface of the tongue. The fibers of the GGp are thicker, stronger, and wider at the base of the tongue than the fibers of the GGa. EMG signals show that the GGa and GGp fibers function differently (Hirose, 1971; Baer et al., 1988). The GGp is more active in high vowels and the GGa is more active in front vowels. The hyoglossus (HG) is a thin quadrangular muscle that arises from the bilateral greater horns of the hyoid bone and runs upward and forward along the lateral side of the tongue. EMG data show that the HG is active for low-back vowels (Baer et al., 1988). The styloglossus (SG) is a long bundle of muscle that arises from the styloid process of the temporal bone, runs forward and downward toward the back of the tongue on each side, and ends at the tongue tip. The SG is active during back vowels. In EMG data, the SG is active for back vowels and most extreme for a high-back vowel /u/. 3. Methods 3.1. Muscle length measurements The lengths of the tongue muscles in mid-sagittal projection were measured on sagittal MRI data on the basis of distance scaling of makers and landmarks, and their variation across vowels was analyzed. 3.1.1. Data collection Sagittal MRI data were collected from four male Japanese subjects during sustained production of the five Japanese vowels /i/, /e/, /a/, /o/, and / /. During the m
HG SG
genioglossus (GGa: anterior, GGm: middle, GGp: posterior) hyoglossus (HGa: anterior, HGp: posterior) styloglossus (SGa: anterior, SGm: middle, SGp: posterior)
m
GG
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MRI scan, the subjects took a supine position, and produced a sustained vowel. Four small spherical capsules of 3 mm in diameter were used as MRI markers. Two markers were used to divide the GG into functional divisions: they were attached to the tongue blade (2 cm back from the tongue tip) and the tongue dorsum (the highest point of the tongue visually recognized with the mouth wide open) along the mid-line. Two others were attached to the tip and nasion of the external nose. Volumetric MRI data were obtained with a clinical MRI scanner (Magnex Eclipse, 1.5 T, Shimadzu-Marconi) with a Fast Spin Echo sequence (4-echo protocol, TE = 15 ms, TR = 2764 ms, NEX = 1). Thirty-five sagittal slices were acquired for each vowel (slice thickness = 2.5 mm, slice gap = 0 mm) with the acquisition time of 3 min. In each slice, the field of view (FOV) was 256 · 256 mm, which was sampled as an image of 512 · 512 pixels. Therefore, the image resolution for each volume is 0.5 · 0.5 · 2.5 mm per pixel. 3.1.2. Procedure for muscle length measurements The intrinsic tongue muscles, extrinsic tongue muscles, and oral-floor muscles such as the geniohyoid and mylohyoid were identified in each slice, and their geometrical patterns were traced manually. All the tracings of the muscles were projected onto the sagittal plane, where muscle length measurements were performed. Therefore, muscle lengths measured in this study represent sagittal projections of their true lengths. The lengths of subdivisions of the genioglossus (GG) were measured directly from the mid-sagittal images because the GG is situated mid-sagittally. The two other extrinsic muscles, the hyoglossus (HG) and styloglossus (SG) were measured from the para-sagittal images. This report describes the results for the three extrinsic tongue muscles. The GG is anatomically arranged as a triangular muscle, and it is reasonable to assume that it has three functional subdivisions, i.e., anterior (GGa), middle (GGm), and posterior (GGp). Each subdivision was defined in the mid-sagittal slice, as shown in Fig. 1. The entire GG in the mid-sagittal plane was bounded by two lines: a line from the mental spine to the tongue blade marker and a line from the mental spine to the upper ridge of the body of the hyoid bone. This region was divided into three subdivisions by two boundary lines, as shown in Fig. 1. The GGa–GGm boundary is the line from the mental spine to the tongue dorsum marker, and the GGm–GGp boundary is the line that extends along the orientation of the short tendon of the GG, which is seen in the image of the GG as a bright line arising from the mental spine of the mandibular symphysis. The length of each subdivision was defined as the length of the line segment bisecting each fan-shaped subdivision from the mental spine to the tongue surface shown by thin white lines with black edges. These bisecting lines were used for measurement because they are geometrically suitable to represent each of the GG’s subdivisions and are less sensitive to the possible subjectto-subject variation in marker placement.
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Fig. 1. Method for length measurement of the extrinsic tongue muscles. ‘‘GG’’, ‘‘SG’’, and ‘‘HG’’ stand for the genioglossus, styloglossus, and hyoglossus muscles, respectively. The letters ‘‘a’’, ‘‘m’’, and ‘‘p’’ mean anterior, middle, and posterior, respectively. A is the turning point of the SG and B is the point of intersection of the SGp and HGa.
The HG is a flat rectangular muscle that arises from the hyoid bone, and merges with the intra-lingual part of the SG at two landmark points (A and B), as shown in Fig. 1. The HG was measured at the anterior (HGa) and posterior (HGp) edges. The length of the HGa is the line segment from the anterior edge of the body of the hyoid bone to Point B, and the length of the HGp is the line segment from the posterior end of the greater horn of the hyoid bone to Point A. The SG is a long thin muscle that arises from the styloid process of the temporal bone, and runs forward and downward to enter the side of the tongue near the mid-pharynx wall to extend toward the tongue tip. In this measurement, the SG was classified into three segments: SGp, SGm, and SGa according to the muscle’s geometrical pattern. The SG posterior (SGp) is the length of the extra-lingual bundle from the styloid process to Point A. The SG middle (SGm) is the length from Point A to Point B, and the SG anterior (SGa) is the length from Point B to the tongue tip, both corresponding to the intra-lingual bundles. 3.2. Measurements of three-dimensional tongue shape Three procedures were employed to describe the threedimensional shapes of the tongue: two were for the measurements of the lateral deformation (width between bilateral arterial landmarks, and sagittal cross-sectional area of the tongue), and the other was for a comparison of the shape of the tongue surface (extraction of the contours of reconstructed three-dimensional surface models). 3.2.1. Data collection One male speaker among the four initial MRI subjects served as a subject for obtaining both coronal and sagittal
S. Takano, K. Honda / Speech Communication 49 (2007) 49–58
slices of the tongue during production of the five vowels. MRI data of a smaller slice thickness of 2.0 mm with no inter-slice gap were collected by using the same MRI scanner with a Fast Spin Echo (4-echo protocol, TE = 15 ms, TR = 3680 ms, NEX = 1, and FOV 256 · 256 mm). For each vowel, 51 sagittal slices and 51 coronal slices were recorded. The acquisition time for each volume was about 4 min. The FOV of 256 · 256 mm was sampled as 512 · 512 pixels, with the image resolution of 0.5 · 0.5 · 2.0 mm per pixel. 3.2.2. Measurements of the distance between bilateral arterial landmarks A sharp bending point of the deep branch of the lingual arteries was identified on each side of the tongue by using the sagittal and coronal images, and the distance between the right and left landmark points was measured to index the lateral deformation of the mid-line tongue tissue (Fig. 4).
3.2.4. Comparison of three-dimensionally reconstructed shapes of the tongue Three-dimensional shapes of the tongue during production of five Japanese vowels were reconstructed as computer graphics models based on the sagittal and coronal MRI slices. The bottom boundary of the tongue model was the bottom of the GGp as described above. The lateral boundaries of the tongue were determined by the lateral surface of the HG for the posterior part of the tongue and by the oral surface of the mandible for the anterior part. 4. Results The MRI data collected for the measurements of muscle lengths and tongue shape changes were found to have sufficient quality for image analysis. All of the four subjects produced the mid-vowel / / regardless of the subjects’ dialects. m
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4.1. Muscle lengths Fig. 2 gives examples of the data from one subject to show the geometries of the extrinsic tongue muscles and bar graphs of muscle lengths for the GGa, GGp, HGa, and SG (three segments) in the five vowels. The mid-sagittal outlines of the tongue for the five vowels show two characteristic patterns of Japanese vowel production. First, while the tongue dorsum is higher in high vowels (/i/ and / /) and lower in a low vowel (/a/), the highest point of the tongue dorsum for / / is not in the rearmost but is between /e/ and /o/. Second, the tongue tip tends to remain m
m
3.2.3. Measurements of the sagittal cross-sectional areas of the tongue Due to volume conservation of the tongue, the lateral deformation of the tongue tissue is also reflected in the change in the mid-sagittal cross-sectional area of the tongue. The outlines of the tongue were manually traced on 11 sagittal slices, and tongue tissue area was measured on each slice. In this measurement, the bottom of the tongue was defined by the bottom margin of the GGp, i.e., a line from the mental spine to the upper ridge of the body of the hyoid bone (see Fig. 1).
Fig. 2. Example of the muscle lengths for one subject. The GGp is shorter in the front vowels /i/, /e/, and, / /. The GGa and GGm are short in the back vowels /a/ and /o/. The HGa is short in the back vowels /a/ and /o/. According to the geometry and length of the tongue muscles, Japanese vowels can be roughly divided into two clusters, a high-front type (/i/, /e/, and / /) and a low-back type (/a/ and /o/). m
m
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similar patterns: The GGm and GGp are longer in /a/ and /o/ (convex lines in the graph), and the GGa, SGm, and HGa are longer in /i/, /e/, and / / (concave lines). This patterning is largely due to the characteristics of the articulation of Japanese vowels having two clusters of front–mid-vowels (/i, , e/) and back vowels (/a, o/). m
m
4.2. Distances between arterial landmarks Fig. 4 shows the locations of the right and left arterial landmarks and their linear distance. The landmarks were defined by an acute curvature of the deep lingual arteries that run between the GG and HG. These curvature points were observed in the lateral aspect of the posterior part of the GGm in this subject. These landmarks move with the tongue tissue, displacing forward in front vowels and backward in back vowels. As shown by the bar graph in Fig. 4, the distance between the two arterial landmarks was smaller in /a/ and /o/, and larger in /i/, /e/, and / /. This result indicates that the mid-line tongue tissue between the bilateral landmarks expands laterally in /i/, /e/, and / /. m
m
4.3. Sagittal cross-sectional areas of the tongue The sagittal cross-sectional areas of the tongue were measured to examine the three-dimensional tongue deformation in vowels. Fig. 5 shows the areas of 11 sagittal sections of the tongue measured for a 2-cm thick segment. In the figure, the areas near the mid-sagittal sections are smaller in /i/, /e/, and / / than those of the lateral sections. In contrast, the areas are nearly constant in all the sections in /a/ and /o/. The volume of the tongue for the measured m
in front even in low-back vowels (/a/ and /o/) and the tongue blade shows a downward indentation, which is called the ‘‘lingual fossa.’’ These characteristics are commonly found in all the subjects. According to the bar graphs, the GGa and GGp show a reciprocal pattern: the GGp is shorter in /i/ and / /, while the GGa is shorter in /a/ and /o/. The HGa and SG tend to be shorter in low-back vowels than in other vowels. The HGa is the shortest in /a/, while the SG is the shortest in /o/. Table 1 lists the measured lengths of the extrinsic tongue muscles for the four subjects for the five vowels and their changes in percentage with reference to the maximum length of each muscle in the five vowels. The GGp and GGm were shorter in /i/ and / /, and the GGa was shorter in /a/ and /o/. The HGa was shorter in /a/ and /o/, while the HGp showed an inconsistent tendency among the subjects. The total length of the SG was observed to be relatively constant compared to the obvious changes in length of the other muscles. The SGa showed individual differences reflecting the positional variation of the tongue tip across subjects. In subject S1, the SGa is longer in /a/ and /o/ than in /i/, /e/ and / /. This is because the tongue tip of this subject remained in front even for back vowels. The SGm was shorter in /a/ and /o/ than in /i/, /e/, and / / for all the subjects. The SGp remained nearly the same both in length and orientation across vowels in all the subjects. Fig. 3 shows the subject-to-subject variation among the measured lengths by plotting the deviation from the mean for each subject according to the data in Table 1. In Fig. 3, the GGp and GGa show the largest changes in length across vowels, while the length of the SGp shows the smallest changes in all the subjects. Some of the muscles show
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m
m
m
m
Table 1 The changes of the muscle length for each vowel in the four subjects GGa
GGm
GGp
SGp
SGm
SGa
HGa
HGp
GGa
GGm
GGp
SGp
SGm
SGa
HGa
HGp
S1
/i/ /e/ /a/ /o/ / / m
47 44 34 33 47
49 51 57 58 55
49 49 51 54 50
51 55 49 47 52
28 28 20 22 23
42 39 50 49 38
38 39 34 30 35
30 28 34 32 31
100.0 93.6 72.3 70.2 100.0
84.5 87.9 98.3 100.0 94.8
90.7 90.7 94.4 100.0 92.6
92.7 100.0 89.1 85.5 94.5
100.0 100.0 71.4 78.6 82.1
84.0 78.0 100.0 98.0 76.0
97.4 100.0 87.2 76.9 89.7
88.2 82.4 100.0 94.1 91.2
S2
/i/ /e/ /a/ /o/ / / m
51 48 41 41 46
56 56 62 66 60
40 47 53 50 45
60 60 59 57 59
28 29 23 22 32
42 42 44 43 42
40 38 33 37 42
31 29 35 35 31
100.0 94.1 80.4 80.4 90.2
84.8 84.8 93.9 100.0 90.9
75.5 88.7 100.0 94.3 84.9
100.0 100.0 98.3 95.0 98.3
87.5 90.6 71.9 68.8 100.0
95.5 95.5 100.0 97.7 95.5
95.2 90.5 78.6 88.1 100.0
88.6 82.9 100.0 100.0 88.6
S3
/i/ /e/ /a/ /o/ / / m
50 44 46 46 50
53 55 58 59 59
43 50 58 58 48
43 42 39 39 42
28 27 21 22 25
36 37 35 32 35
35 32 27 27 33
28 28 33 35 30
100.0 88.0 92.0 92.0 100.0
89.8 93.2 98.3 100.0 100.0
74.1 86.2 100.0 100.0 82.8
100.0 97.7 90.7 90.7 97.7
100.0 96.4 75.0 78.6 89.3
97.3 100.0 94.6 86.5 94.6
100.0 91.4 77.1 77.1 94.3
80.0 80.0 94.3 100.0 85.7
S4
/i/ /e/ /a/ /o/ / /
47 48 34 35 45
59 56 62 66 59
45 53 62 59 47
46 47 47 46 46
32 32 28 28 32
43 43 47 45 43
28 25 22 23 28
31 28 22 25 27
97.9 100.0 70.8 72.9 93.8
89.4 84.8 93.9 100.0 89.4
72.6 85.5 100.0 95.2 75.8
97.9 100.0 100.0 97.9 97.9
100.0 100.0 87.5 87.5 100.0
91.5 91.5 100.0 95.7 91.5
100.0 89.3 78.6 82.1 100.0
100.0 90.3 71.0 80.6 87.1
m
Left: length in mm, right: length in percent relative to the maximum length of each muscle.
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15 10 5 0 /i/
/e/
/a/
/o/
/m/
Landmark distance [mm]
Fig. 3. A graph of the muscle length deviation from the mean for the five vowels.
Fig. 4. The distance between the right and left landmark points. Each landmark is the sharp curve of the deep branch of the lingual artery.
segment (the area under the curve in the figure) is larger in /a/ and /o/, and smaller in /i/, /e/, and / /. The contrast of the Japanese vowel clusters is found in the area measurement. The result of the area measurement among the eleven 2cm thick sagittal slices of the tongue indicates that vowel articulation involves three-dimensional tongue deformation. The smaller mid-sagittal areas in /i/, /e/, and / / indicate that the mid-line tongue tissue is sagittally compressed to expand laterally. The tissue pushed aside from the midline possibly contributes to bulging the tongue tissue on both sides of the mid-line groove in these vowels. In contrast, the larger mid-sagittal areas in /a/ and /o/ explain that the tongue tissue is compressed medially toward the mid-line to expand sagittally. The tongue tissue gathered
in from the sides contributes to forming a bunched shape of the tongue dorsum in /a/ and /o/. 4.4. Three-dimensional reconstruction of the tongue surface Fig. 6a shows the front views of the tongue models for the three vowels /i/, /a/, and /o/. The geometry of the extra-lingual part of the SG is nearly constant across vowels despite the obvious changes in the whole tongue shape. The asymmetry of the tongue shape is extreme in /i/ but less obvious in /a/ and /o/. The asymmetry of the tongue in /i/ reflects the asymmetrical shape of the palate and causes the off-mid-line deviation of the front part of the vocal tract. In evidence, the MRI data show that the right roof of the hard palate is higher than the left roof in this
m
m
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Fig. 5. Result of tongue area measurement in the mid-sagittal plane and five para-sagittal planes for both sides. The 11 slices correspond to the tissue segment of 2-cm thickness. The right-top shows the mid-sagittal plane of MRI with the measured area of the tongue. The right-bottom indicates the measured portions that were seen from the front.
Fig. 6. (a) The front view of the three-dimensional tongue model in /i/, /a/, and /o/. The geometry of the SG is almost constant, while the insertion point of the SGp moves slightly possibly due to jaw movement. (b) The top view of the three-dimensional tongue model in /i/, /a/, and /o/. As the tongue moves forward and backward, the shape of the pharyngeal surface of the tongue changes, from concavity in front vowels to convexity in back vowels, whereas the SG maintains the same geometry. The tongue blade markers disappear in the 3D reconstructions of the tongue for /i/ and /a/ due to the downward chemical shift of the markers on the original images and surface rendering for the reconstructions.
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subject. The lateral width of the tongue varied minimally, even though considerable deformation of the internal tongue tissue was observed. Fig. 6b shows the top views of the vowels /i/, /a/, and /o/. The pharyngeal surface of the tongue is posterior in /a/ and /o/, and front in /i/. Nonetheless, the geometry of the SG does not vary much and the location of the insertion point of the SG to the tongue is nearly constant in all the vowels. Also, the tongue width does not differ greatly regardless of the position of the tongue body. The pharyngeal surface of the tongue is convex forward in /i/, whereas it is convex backward in /o/. These observations suggest that the back positions of the tongue for back vowels are caused principally by internally accumulated strain rather than by external traction force. 5. Discussion In this study, the lengths of the extrinsic tongue muscles were measured and the three-dimensional deformation of the tongue tissue was visualized for the five Japanese vowels. In this section, the results obtained are compared with previous descriptions in the literature, and the roles of the extrinsic tongue muscles in vowel articulation are discussed. 5.1. Functions of the extrinsic tongue muscles 5.1.1. The genioglossus (GG) The GG is the largest and strongest muscle in the tongue (Abd-el-malek, 1939; Gray et al., 1858/1989; Takemoto, 2001). Due to its triangular configuration, this muscle can generate basic patterns of tongue deformation for vowels by a combination of contraction and relaxation among the subdivisions. The vowel-to-vowel changes in muscle length of the three subdivisions of the GG were the most extensive among the extrinsic tongue muscles, as shown in Fig. 3. These observations suggest that the GG plays a dominant role in determining the tongue shape in vowel articulation. The GGa is defined in this study as a subdivision of the GG that runs vertically. The GGa was shorter in back vowels than in front vowels. This suggests its contribution to tongue backing and bunching in /a/ and /o/. Supposing that the tongue tissue behaves as a hydrostat, the shortening of the GGa should result in a backward bulging of the tongue body, when the GGm and GGp are inactive. The tongue blade in the mid-sagittal MRI slice often shows the concavity of the lingual fossa, which also supports the role of the GGa in back vowel articulation. These characteristics are typical for Japanese back vowels. Considering the sparse arrangement of muscle bundles in this region, however, this effect alone may not be sufficient to complete the shape of the tongue for back vowels. The GGm is an oblique muscle bundle in the tongue, and it was shorter in front vowels than in back vowels in the result of our muscle length measurement (see Table 1 and
Fig. 3). The contraction of this bundle alone can compress the mid-sagittal tongue tissue to form an oval cross-section, driving the free tongue blade forward and the root downward. Since the downward force is limited by the mass beneath (i.e., the GGp), contraction of the GGm is thought to advance and elevate the front part of the tongue. The GGm in this study can be estimated to correspond to the ‘‘GGa’’ in the EMG study by Baer et al. (1988), because they divided the GG into two portions (GGa and GGp). The GGp is the largest subdivision among the GG because it spreads laterally toward the tongue root (Abd-el-malek, 1939). This muscle is known to be most active in high vowels /i/ and /u/, and produced the largest signals in the EMG recordings of Baer et al. (1988). The contraction of this bundle advances the tongue root, elevating the tongue tissue above, and thus forms the tongue shape of high vowels. In the MRI data, the length of the GGp changes dramatically across vowels, which suggests that the GGp is the muscle that most strongly determines vowel articulations. It is reasonable to speculate that contraction of the GG causes lateral expansion of the internal tissue of the tongue, as seen in the variation of the distance between arterial landmarks. The deep lingual artery runs along the lateral margin of the GGp and GGm, and the contraction of these muscles should produce a change in the width of the muscle bundles. As evidence, in the front vowels /i/ and /e/, the area of the mid-sagittal cross-section was smaller than the area of the para-sagittal sections. This implies that the GGp and GGm together produce a high-front position of the tongue body while expelling the mid-line tongue tissue to the sides. 5.1.2. The hyoglossus (HG) The HG is a thin quadrangular muscle, which runs along the lateral backside of the tongue, and blends with muscles such as the SGm in the sides of the tongue. The length of the HG was measured in two portions, i.e., anterior (HGa) and posterior (HGp). The length of the HGa was shorter in back vowels than in front vowels, which agrees with the EMG data (Baer et al., 1988) as well as with the general assumption that the activation of the muscle causes a low-back position of the tongue body. The tongue model simulation by Perkell (1996) reports that the HGp was more dominant than the HGa for producing tongue dorsum bulging for the English back vowel /a/. The disagreement between Perkell’s and the present studies may be due to language-specific differences in vowel articulation: the tongue dorsum for /a/ is lower in English while it is higher for /a/ in Japanese. 5.1.3. The styloglossus (SG) In this study, the SG was defined as three segments: two intra-lingual (SGa and SGm) segments and one extra-lingual (SGp) segment. It has been believed that the SGp draws the tongue body obliquely toward the direction of the styloid process (Baer et al., 1988), being antagonistic
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to the oblique bundle of the GG (GGm) (Maeda and Honda, 1994; Honda, 1996). While this assumption is natural from a lateral view of this muscle in anatomical literature, it differs from the result of our measurements due to the following three reasons. First, the length and orientation of the SGp are nearly unchanged across vowels as seen in Figs. 2 and 3. This observation may in part be due to the fact that the SGp is surrounded by soft tissue (Gray et al., 1858/1989), which allows muscle shortening with no obvious displacement of the muscle in the direction perpendicular to its axis of contraction. Even in the case that the SGp slides in the conduit formed by a soft tissue wall as it contracts, the direction of the force to pull the tongue might not be toward the styloid process, but simply backward. Second, the insertion point of the SG on the tongue body is too low to form the extreme high-back position of the tongue as seen in the English /u/ by directly pulling the muscle’s insertion point. Third, the shape of the pharyngeal surface of the tongue viewed from the top is convex backward in /o/ and /a/ (see Fig. 6a), implying an effect of intra-lingual deformation to form a bunched shape of the tongue. Perkell (1996) also notes the function of the intra-lingual segment of the SG to produce upward bulging of the tongue dorsum. All of these observations suggest that a back vowel articulation is not primarily caused by traction forces of the SGp and HG but by tissue stress generated by several muscles, such as the SGm, GGa and HGa. In addition, the contraction of the transverse muscle and the relaxation of the GGp may also be involved in the production of low-back vowels. 5.2. Vowel production mechanisms Physiological mechanisms for producing Japanese vowels are proposed below based on the results of the MRI analysis regarding the geometries of the extrinsic tongue muscles. 5.2.1. High-front vowels The GG is the dominant muscle in determining the tongue shape for vowels. When the entire GG is activated, the GGp powerfully draws the tongue root forward, and advances the tongue body. The contraction of the GG also results in the lateral expansion of the mid-line tongue tissue and the compression of the lateral tissue. Since the lateral aspect of the tongue is limited by the mandible, the whole tongue is raised toward the palate due to the internal strain mainly effected by the GGp. The GGp has a major effect on the entire tongue, because it has the largest volume and can apply the greatest force. The GGa can be lengthened passively even if it is active (Baer et al., 1988). The GGa maintains the mid-line groove of the tongue and maintains a small front cavity for front vowels. 5.2.2. Low-back vowels In back vowels, the GGp and GGm are relaxed to allow the tongue shape and position to be controlled by other
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muscles. The SGm contracts for back vowels and helps form the tongue into a bunched shape. In low-back vowels, the HGa contracts to lower the tongue and make it bulge out in a position and direction. The GGa contributes to lowering the tongue blade or making the anterior mid-line groove, which also helps keep the unique shape of the tongue blade with the lingual fossa in Japanese vowels. Contrary to previous accounts, the SGp alone cannot draw the tongue body in the direction of the styloid process, but the SGm contributes to forming the bunched shape of the tongue body for back vowels.
6. Conclusion The tongue muscles make the most significant contributions to the articulation of speech sound. Despite their critical functions supporting human communication, knowledge of how each muscle performs its role has mostly been speculative. While EMG and anatomical data have been the major source of information for estimating the physiological function of the tongue muscles, these data do not provide direct evidence of the causal relationships. This is because the tongue muscles blend with each other and interact mutually in a three-dimensional manner. Visualization of muscle bundles during static speech gestures, as described in this study, should supplement our understanding of the functions of the tongue muscles and the mechanisms of speech production, even though measurements of muscle geometry do not directly index muscle activity but reflect an equilibrium state among the muscles. The results from this study that aimed at revealing the physiological mechanisms of vowel production, are summarized as follows: (1) The entire genioglossus (GG) demonstrated the greatest change in length across vowels, suggesting that it plays a dominant role in producing front vowels by contraction and assists back vowels by relaxation. In comparison, the styloglossus (SG) and the hyoglossus (HG) show a smaller extent of length changes, inferring a less deterministic role in vowel production. (2) The SG was previously thought to pull the tongue in the direction of the styloid process, which was not supported by the present study because the extra-lingual part of the muscle (SGp) remained constant in length and orientation. Instead, the intra-lingual part of the muscle (SGm) appears to be the most critical for the bunching of the tongue in a backward direction. (3) The tongue shape for vowel production is caused by the internal deformation of the tongue tissue. In front vowels, the internal tissue of the tongue is expanded laterally. In back vowels, the tongue is compressed medially.
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Acknowledgements We are grateful to Dr. Shinobu Masaki, Mr. Yasuhiro Shimada, and Mr. Ichiro Fujimoto at the ATR Brain Activity Imaging Center for their help in conducting this study. We also thank Dr. Masaaki Honda at Waseda University for his effort in realizing the work as the P.I. of the CREST project. This work was conducted while the first author belonged to the CREST of JST (Japan Science and Technology). This research was also supported in part by the National Institute of Information and Communications Technology. References Abd-el-malek, S., 1939. Observations on the morphology of the human tongue. J. Anat. 73, 201–210. Baer, T., Alfonso, P.J., Honda, K., 1988. Electromyography of the tongue muscles during vowels in /EpVp/ environment. Ann. Bull. RILP 22, 7– 19. Engwall, O., 2003. Combining MRI, EMA and EPG measurements in three-dimensional tongue model. Speech Commun. 41 (2–3), 303–329. Gray, H., Williams, P.L., et al. (Eds.), 1989. Gray’s Anatomy, 37th ed. Churchill Livingstone, Edinburgh (Original work published 1858). Hirose, H., 1971. Electromyography of the articulatory muscles: current instrumentation and technique. Haskins Laboratories Status Report SR-25/26, pp. 73–86.
Honda, K., 1996. Organization of tongue articulation for vowels. J. Phonetics 24, 39–52. Kakita, Y., Fujimura, O., Honda, K., 1985. Computation of mapping from muscular contraction patterns to formant patterns in vowel space. In: Fromkin, V.A. (Ed.), Phonetic Linguistic. Academic Press, New York, pp. 133–144. Maeda, S., Honda, K., 1994. From EMG to formant patterns of vowels: the implication of vowel spaces. Phonetica 51, 17–29. Masaki, S., Tiede, M., Honda, K., 1999. MRI-based speech production study using a synchronized sampling method. J. Acoust. Soc. Jpn. (E) 20, 375–379. Miyawaki, K., 1973. A study on the musculature of the human tongue – observations on the transparent preparations of serial sections. Ann. Bull. RILP 8, 23–49. Perkell, J.S., 1996. Properties of the tongue help to define vowel categories: hypotheses based on physiologically-oriented modeling. J. Phonetics 24, 3–22. Stone, M., Lundberg, A., 1996. Three-dimensional tongue surface shapes of English consonants and vowels. J. Acoust. Soc. Amer. 99, 3728– 3737. Stone, M., Davis, E., Douglas, A., 2001. Modeling the motion of the internal tongue from tagged cine-MRI images. J. Acoust. Soc. Amer. 109, 2947–2982. Takemoto, H., 2001. Morphological analyses of the human tongue musculature for three-dimensional modeling. J. Speech Lang. Hear. Res. 44, 95–107. Wilhelms-Tricarico, R., 1995. Physiological modeling of speech production: methods for modeling soft-tissue articulators. J. Acoust. Soc. Amer. 97, 3085–3098.