Range exploration of phonation and pitch in the first six months of life




Bettany, Lisa Danielle.

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In the first six months of life, infants systematically explore the laryngeal parameters of phonation and pitch. In existing research, laryngeal vocalizations, defined as "vegetative" or "reflexive", are characterized by the presence of "strained" and "rough" phonation with "high degrees of vocal tension" and dynamic pitch variations. Previous studies have focused exclusively on the development of linguistic precursors, including only "speech-like" sounds with "normal" or modal phonation. These studies have excluded laryngeal vocalizations (i.e. grunts, squeals and growls) from their experimental analyses and therefore have not provided an accurate description of early phonetic development. This thesis attempts to fill the gap in the phonetic and articulatory description of the infant vocal capacity by investigating the exploration and development of the laryngeal mechanisms involved in the production of laryngeal phonation and laryngeal pitch. In order to account for the productive capability of infants, it is necessary to consider the vital role of the primary articulator in the adult and infant larynx, the aryepiglottic laryngeal sphincter. The mechanism of the laryngeal sphincter is actively engaged in early infancy to protect the tracheal airway from inundation. In this study, two quantitative analyses of one English-speaking infant's vocalizations in the first six months of life were conducted. In analysis one, auditorily based analysis of 824 vocalizations was performed using the phonetic taxonomy of laryngeal modalities developed by Esling and colleagues (Esling, Benner & Bettany, 2004a; Esling, 2002). The incidence of five phonatory settings (i.e. harsh voice, creaky voice, whisper, modal voice and falsetto) and three pitch levels (low, mid and high) was reported. In analysis two, the laryngeal parameters involved in "range exploration", defined in this study as the instance of within-vocalization phonatory altemations, were quantified by means of acoustic analysis. 120 randomly selected vocalizations (20 from each of the six months) were used in this analysis component. The durations of the vocalizations and of individual phonatory settings within each vocalization were calculated using spectrographic analysis and compared statistically. The present study was able to accurately identify the phonetic range and productive repertoire of infant vocalizations produced in the first six months of life. Four main findings were reported in this study: (1) the default setting in early infancy is laryngeally constricted and low-pitched, (2) the infant's phonetic repertoire of phonation and pitch expands at four months (3) the incidence of within-vocalization phonatory altemations increases at four months and, (4) the productive integration of phonation and pitch is acquired by the sixth month of life.