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Abstract:
We have investigated the acoustic effects of a profound
modification of the laryngeal anatomy and its consequent
shortening of the vocal-tract length after supracricoid partial
laryngectomy (SCPL) with cricohyoidoepiglottopexy (CHEP).
SCPL-CHEP is a maximal conservation technique for glottic
carcinoma, removing the whole thyroid cartilage and paraglottic
space including the vocal folds. The cricoid cartilage, the two
mobile arytenoid cartilages, the hyoid bone and the suprahyoid
epiglottis are conserved. This surgery allows the conservation of
the basic functions of the larynx (respiration, swallowing and
phonation). In the absence of the vocal folds, voice is produced
by the formation of a "neoglottis" by the inward and forward
movement of the arytenoid cartilages toward the remnant of the
epiglottis. As a consequence of this, the length of the
vocal-tract back cavity is expected to be shortened by
approximately 3 cm. In order to evaluate its acoustic
consequences, we tracked the three first formant frequencies of
the cardinal vowels /a/, /i/ and /u/. Ten male patients were
recorded before surgery and at six, 12 and 18 months after
surgery. The results were compared with those of 10 male normal
speakers having the same range of age. Six months after surgery,
all the formants of the vowel [a] shifted up to higher
frequencies and remained high even at 18 months after operation.
For the vowel [i], F2 steadily lowered after operation. For the
vowel [u], F1 and F2 were within normal limits and remained
stable. In seeking an articulatory explanation of formant
deviations, the articulation of the vowels [a] and [i] uttered by
two of the 10 patients were observed using cinefluorography at 24
months after surgery. The observation confirms that the
neoglottis is indeed formed at the level of the epiglottis. The
back cavity corresponds to the pharynx alone. In the case of [i],
the neoglottis is formed by strongly retracting the tongue root
and the epiglottis against the arytenoid cartilages. This
tongue-root retraction appears to be a key maneuver for the
formation of the neoglottis and then for voicing. The tongue-root
retraction seems to be the cause of an extra fronted position of
tongue blade, which results in a relatively long cavity behind
the constriction and then in the observed low F2 values. In the
case of [a], we didn't find any particular compensatory maneuver.
This is expected, since acoustic theory dictates the
impossibility of simultaneously lowering the F1 and F2
frequencies of [a] by articulatory adjustments to compensate the
shortened tract. In summary, we report here an example of
articulatory compensation to produce perceptually adequate vowels
under the unusual phonatory constraint and in turn demonstrate a
high adaptive capability of the human speech production
system.
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