An MRI Study of Swedish Fricatives : Coarticulatory effects

Five Swedish fricatives have been analysed in different vowel contexts using Magnetic Resonance Imaging (MRI). The fricatives were acquired for sustained articulations in VCV context both in the midsagittal plane and in full 3D for one speaker of Swedish. The MR images were used for threedimensional reconstructions and extraction of the vocal tract outline in the midsagittal plane. This allowed for coarticulatory studies based on the area functions and the articulatory measures determined from the vocal tract contours. Coarticulatory effects have been found for the pl ace of constriction, the protrusion, the jaw and larynx height and the cross-sectional area. Synergetic coupling are also found between the jaw advancing and lip protrusion and between the larynx height and lip protrusion. Figure 1. The 3D data acquisition grid, consisting of three series of 18 parallel slices: A) horizontal (axial) slices fro m below the glottis to the top of the pharynx. B) Slices inclined at 45° in the bent part of the vocal tract. C) Vertical (coronal) slices from the soft palate to in front of the lips. The grid is superimposed on the midsagittal image of / ,V,/. 2.2 Image processing 2.2. 1 The midsagittal set The midsagittal contour for each configuration (see Fig. 2) was determined using a version of BtoC ([6]) adapted for MRI. The image was thresholded in binary mode to automatically detect the air-tissue boundary. Using this boundary as a guide, the 10 sub-contours of the upper lip, palate, velum, pharynx, back larynx, lower lip, jaw, tongue, front larynx and glottis were extracted manually using a Matlab-software developed at ICP. These sub-contours provided a number of articulatory parameters that have been studied and used in a linear midsagittal articulatory model ([4]). The results from the midsagittal parameter evaluation will be used in the KTH 3D vocal tract model ([7]). 2.2.2 The 3D set The contour of the vocal tract was extracted from each image using edge detection in thresholded images. Each contour was manually checked, and when needed, corrected by moving, adding or removing the Bezier control points of the contour. The classical MRI airway segmentation problems were handled as follows: 1) the piriform sinuses and the sublingual cavities were regarded in an image only when they were connected with the main air passage; 2) the volume occupied by the teeth was removed in the reconstruction process by fitting the vocal tract shape and the dental cast to the common reference system; 3) when the epiglottis was present, the outline of the vocal tract was delimited by the closed contour consisting of the epiglottis and the pharynx wall; 4) the vocal tract contour extraction at the lips were pursued as long as the lips determined a closed contour. 2.3 3D reconstruction The vocal tract shape was reconstructed from the contours using a semi-polar partially dynamic grid of 37 gridlines as defined by Beautemps et al. [8]. It consists of a fixed central polar grid connected to two linear grids of variable length; one attached to the tongue tip and the other to the glottis. The grid hence follows movements of the larynx, the tongue tip and the lip protrusion dynamically. The hard palate of every configuration was aligned with a fixed common palate shape that serves as reference for the grid in the 3D reconstruction. Each planar contour was finally re-sampled with a fixed number of evenly distributed points. The contours were then rendered as shown in Fig. 3, allowing for comparisons between different vowel contexts. Qualitative comparisons of vocal tract shape can be made between different fricatives and vowel contexts (refer to [4] for vocal tract shapes of other Swedish fricatives than / V/). As was evidenced also by a study of English fricatives [9], the difference in the vocal tract shape is not limited to the place and amount of constriction. The areas of minimum supraglottal constriction are smaller than for the English subjects in [9], but this cannot be attributed to language characteristics based on a single subject’s data. Oral as well as pharyngeal volumes differ sometimes greatly between fricatives and vowel contexts, illustrating the value of 3D measurements and modeling. a) Vowels b) /V/ c) /I/ d) /e/ e) // f) /Í/ Figure 2. Midsagittal contours of isolated vowels and of fricatives in vowel contexts / D/ (solid line), / ,/ (dashed), / R/ (dashdotted) and / X/ (dotted). The main coarticulatory effects are the lip protrusion and the larynx height, but some context influence can also be seen on the tongue contour and the jaw height and advancing. a) /DVD/ b) /,V,/ c) /XVX/ Figure 3. The vocal tract shape for / V/ in three different vowel contexts. Coarticulation influences the place and the length of the constriction, the lip protrusion and mouth opening, but also oral and pharyngeal volumes. 3 COARTICULATORY EFFECTS 3.1 The midsagittal set The articulatory measures of the jaw, tongue, lips and larynx positions or shapes were studied with respect to coarticulation. 3.1. 1 Constriction The degree of constriction, i.e. the midsagittal distance at the narrowest air passage, is to some extent influenced by coarticulation. The openness of the surrounding vowel (Fig. 4a) determines the degree of constriction for the fricative, as shown by Fig. 4b). The constriction is narrower in / ,/ context than when the fricative is surrounded by open, rounded vowels. The range in midsagittal distance is 0.2-0.25 cm for / V/ and /Í/, whereas the range is smaller, but still observable for / /. For /e/, the coarticulatory influence is rather on the place of constriction, the range being 0.5 cm from the front-most (/ ,/) to the back-most (/ X ) vowel context. The context also influences the place of constriction for /V/ and /Í/. For the alveolar fricative / V/, it is advanced in rounded vowel context, due to jaw protrusion. For the velar fricative /Í/ it is retracted for / R/ and / X/, following the tongue backing. As expected, Fig. 4 shows no coarticulatory influence for /I/. 3.1.2 The Jaw The jaw height (Fig. 5) is relatively constant with respect to vowel context for the four front fricatives, indicating that a rather closed jaw is required to achieve the constriction. The velar fricative / Í/ allows for more coarticulation, since its place of constriction is less influenced by the jaw opening, and the jaw height consequently becomes much larger in open vowel context (more than 1 cm larger in /R/ than /,/ context), as indicated also by Fig. 2f. The jaw advancing (Fig. 5) is influenced by coarticulation in all contexts and two interesting phenomena are present. 1) The jaw moves in synergy with the lips to achieve lip protrusion in rounded vowels (a difference of 0.3-0.4 cm comparing / ,V,/ to /RVR/ or /,Í,/ to /XÍX/). 2) The coarticulation on /I/ is the opposite; the jaw is retracted in rounded vowel context in order to achieve tongue backing. 3.1. 3 Vocal tract length: lip protrusion & larynx height The vocal tract is lengthened at both ends simultaneously in rounded vowel context (Fig. 6, solid lines), in agreement with the findings of e.g. Hoole & Kroos ([10]). The most important part of the lengthening is due to the coarticulatory effect on the lip protrusion (Fig. 6, unconnected series), with 0.6-1 cm larger protrusion in / R/ and / X/ context than for / , , except for /RIR/, that is constrained by the place of constriction. The coarticulatory contrast is increased by the larynx height (the larynx is 0.1-1.2 cm lower in / X than in / ,/ context), but compared to the lip protrusion it is influenced less by the vowel and more by the fricative. Note that the vocal tract lengthening due to larynx lowering in Fig. 6 is measured relative that of / , , having the highest larynx position.