Morphological Variation in the Adult Vocal Tract : A Study Using rtMRI

No two vocal tracts are exactly identical. Human vocal tracts display considerable morphological variation in terms of size, proportions and shape. Previous studies have focused on the dramatic variations present during development [1], [2], [3]. However, substantial differences also exist between fully developed individuals in terms of size, shape and proportions. We describe and quantify broad differences in morphology, explain certain systematic aspects in the variation (e.g., sexual dimorphism) and review some relevant characteristics of vocal tract structures. Quantifying morphological variation is motivated by both practical and theoretical considerations. It has long been known that morphological differences affect acoustic output by altering the resonant properties of the vocal tract. Growing evidence also suggests that morphology affects production behavior. For example, palate shape explains certain differences in the production of sibilant fricatives [4], [5]. We ultimately hope to predict individual production patterns from observations of morphology. In addition, the utility of articulatory data (e.g., pooling data across subjects) is limited by poor understanding of morphological variation. Several studies have used general warping functions to impose normalization of various vocal tract landmarks [6]. We believe that a firm understanding of the space of variations will facilitate a normalization scheme which is more meaningful. We collected realtime magnetic resonance imaging (rtMRI) data of 30 adult subjects (11 female, 19 male) from diverse linguistic (15 English, 8 German, 5 Mandarin, 1 Hindi) and racial (24 caucasian, 6 asian) backgrounds. Subjects spoke while lying supine in the scanner. The use of rtMRI allows subjects use their articulators naturally (i.e., there is no posing of the articulators). Images were reconstructed at a resolution of 68×68 pixels. Further details of our acquisition protocol can be found in [7]. For each subject, we identified images representing absolute rest position during breathing, with clenched teeth. We used Canny edge detection [8] with manual linking and correction to trace the hard Fig. 1: Hard structure traces and anatomical landmarks for one midsagittal vocal tract image. structures of the vocal tract, including (1) along the chin, and (2) along the passive articulators inside the vocal tract. The latter trace followed the upper dentition, the maxilla and palatine bones, along the posterior surface of the vomer, the pharyngeal tonsil and down the pharyngeal wall to the thyroid cartilage. Several anatomical landmarks were identified for the purposes of analysis (see Fig. 1). Fig. 2: Distribution of the vocal tract proportions, corresponding to the segments shown in Figure 1. We calculated the length between each landmark as a proportion of the entire trace length. The distributions of these proportions for the interior trace are shown in Fig. 2. The distributions are highly unimodal, with the pharyngeal wall segment making up the nearly half of the entire length. Interestingly, pharyngeal wall length is proportionally longer in males than in females to a statistically significant degree (Student’s t-test, p = 0.045). There was also a significant difference between chin proportions by