Physiological Factors Causing Tonal Characteristics of Speech: from Global to Local Prosody

Voice fundamental frequency (F0) determines the tonal quality of vowels, and its rise and fall comprise part of prosody in speech. This seemingly simple linear function results from highly complex physiological factors and thus lacks definitive explanations of the causal mechanisms. This report reviews previous studies and recent discoveries regarding the causal factors of F0 patterns and discusses possible explanations for lexical accent, local F0 fluctuations, and phrasal declination. A special focus is placed on the following topics. (1) Historical arguments on the two actions of the cricothyroid joint, rotation and translation, for stretching the vocal folds: whether they both actually exist and how they contribute to F0 patterns is revisited with MRI observations. (2) Causal mechanisms of the so-called micro-prosody, i.e., F0 fluctuations due to voicing and vowel articulation: whether such local prosodic patterns are automatically derived from the relevant anatomical structure or derived from deliberate efforts of a speaker to enhance speech perception is discussed based on EMG data.