The Role of Vocal Ligament in Fundamental Frequency and Adduction Control

This project investigates the anatomical and biomechanical bases that set the upper limits of mammalian fundamental frequency (f0) in self-sustained vocal fold vibration. Traditional models attribute control of f0 primarily to changes in vocal fold length and tensile stress in muscle and ligament tissues; intrinsic laryngeal muscles alter fold length and adjust tension in muscle and ligament layers to generate a wide f0 range. Empirical data and modeling, however, indicate an apparent upper limit—approximately 1600 Hz in humans—that cannot be explained by ligament and muscle stress alone. The work documents examples of human whistle voice sequences with fundamentals well above this threshold (e.g., tones around 2.4 kHz, glide sequences up to 5.5 kHz) and comparable extreme f0 values in non-human mammals (e.g., elk calls to 2.4 kHz; dolphins, bats, and some rodents producing calls into the kilohertz range), highlighting a mechanistic gap in current theory.

To address this gap, the authors propose the epithelial dominance hypothesis: at very high f0, thin tissue layers near the epithelial surface of the vocal fold become the primary contributors to elastic restoring forces that sustain vibration. Anatomical evidence shows weak allometric scaling of epithelial thickness and cell-layer number across species, which could allow epithelial-dominated mechanisms to produce frequencies outside the size-dependent spectral range predicted from vocal fold length alone. Computational simulations using available tissue-property data support the plausibility of an epithelium-based oscillator: epithelial cells together with collagen fibers in the lamina densa could form structures capable of generating fundamental frequencies in the kilohertz range with very shallow depths of vibration.

The study situates the epithelial dominance idea within established frameworks such as the body-cover theory of vocal fold vibration and links it to pedagogical and mechanistic register distinctions (M1, M2, M3). The authors suggest these three vibration mechanisms may correspond to dominance by muscle, ligament, or epithelium, respectively. They note that modeling extremely high f0 as ligament- or muscle-dominated would require fiber stresses an order of magnitude higher than recorded experimentally, underscoring the need for an alternate or complementary tissue mechanism.

Empirical approaches reported include a morphometric analysis of vocal fold epithelium across 21 mammalian species spanning a wide body-size range (from 30 g mice to 5,000 kg giraffes), revealing substantial diversity in epithelial morphology. Computational analyses modeled an epithelium coupled to a lamina densa and portions of superficial lamina propria, demonstrating conditions under which epithelial-layer mechanics could support high-frequency self-sustained oscillation. The authors acknowledge a lack of direct stiffness measurements for vocal fold epithelium and the experimental challenges in obtaining such data, and they draw on mechanical properties of other epithelial tissues and observed phenotypic diversity to build their case.

Overall, the work reframes how the vocal ligament’s role in f0 control is understood: ligament and muscle explain much of the conventional f0 range, but epithelial dominance may be necessary to account for extremely high frequencies and potentially different adduction-related control regimes. By integrating morphometrics, computational modeling, and comparative examples, the project proposes a testable mechanism that extends current voice-production theory and guides future empirical measurements of epithelial mechanical properties and their interaction with ligament and muscle layers.