The Scope and Limits of Iconic Prosody: Head Angle Predicts f0 Changes While Object Size Effects Are Absent

The relation between the fundamental frequency of the voice (f0) and vertical space has been shown in previous studies; however, the underlying mechanisms are less clear. This study investigates the relationship between head angle and f0 in iconic prosody, along with the influence of object size on lip opening and formant frequencies. In the experiment, participants pointed to objects of two different sizes and in various vertical positions while saying the words "piff" or "paff," which induced vertical head position change. Head angle emerged as a reliable predictor of f0, with a larger angle increasing the f0. This effect was consistent despite individual variations in head movement. While the vertical position of the object also showed a reliable effect on f0, head angle substantially outperformed it as a predictor, suggesting that head angle represents the primary physiological mechanism predicting f0 changes. Conversely, object size did not predict either lip opening or formant dispersion. Lip opening and formant dispersion were purely indexical, tracking vowel-specific articulatory configurations rather than external object properties. These findings underscore the role of head position in modulating f0 through direct physiological coupling, potentially underpinning iconic prosody, while revealing the limits of size-related iconicity in parameters constrained by phonemic requirements.