An MRI study of the physiological mechanism of F0 lowering

Abstract

The phenomena of F0 lowering in utterances, widely observed in a word accent or near a phrase boundary, is not sufficiently explained by the relaxation of F0 raising musculature, and the physiological mechanism for F0 lowering has been a question to be examined. Morphological observation of laryngeal framework was performed in this study using the magnetic resonance imaging (MRI) during phonation. Midsagittal images near the larynx were taken in different F0 levels in four male speakers. Relative positional changes among laryngeal components were measured to explore the chain of mechanisms that allows vocal fold shortening in low F0. In the results, vertical movements of the larynx were consistently observed to be correlated with F0 levels. Movements of the hyoid bone and changes in laryngeal cavity size were also found. A rotation of the cricoid cartilage was always associated with laryngeal descent towards lower F0. This rotation occurred to the direction of shortening the vocal folds, and its mechanism was confirmed by vertical sliding motion of the posterior plate of the cricoid cartilage along the physiological curvature of the cervical vertebrae. This mechanism can account for the well-known tendencies of laryngeal descent and strap muscle activity in F0 lowering.