An MRI study on the relationship between oral cavity shape and larynx position

Individual variation of larynx position reflects human morphological differences and thus contributes to generating biological information in speech sounds. This study examines the factors of orofacial morphology that co-vary with larynx position based on MRI data collected for 12 Japanese and 12 English speakers. The materials are midsagittal craniofacial images, and the method is based on the measurement of angles and indices. Among all the measures examined, the aspect ratio of the oral cavity showed the highest correlation (r=0.87) with a larynx height index (ratio of arytenoid apex palatal plane distance and anterior nasal spine nasopharyngeal wall distance), and a facial angle (angle of maxillary incisor nasion nasopharyngeal wall) showed the second highest correlation (r=0.66) with the larynx height index. Since the facial angle is known to be an index of prognathism, the results suggest that larynx position co-varies with oral cavity shape, being higher when the oral cavity develops more forward. This tendency i s parallel to the general relationship between the oral shape and larynx position among mammals, and provides a reliable basis for estimation of larynx position in fossil humans.