Supralaryngeal Vocal Tract
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Humans have a species-specific tongue. Half of the human tongue is positioned in the mouth, half in the throat where it forms the forward (anterior) margin of the pharynx. The result is a supralaryngeal airway (supralarygngeal vocal tract – SVT) that has a midpoint right angle bend and a horizontal (SVTh) and vertical (SVTv) segments that are almost equal in length. The human vocal tract is able to form “quantal” speech sounds such as the vowels [i] and [u] ( the vowels of the words see and too) that enhance the process of speech perception and that are insensitive to slight errors in articulation. Moving the tongue, upwards and forwards without changing its shape can yield the abrupt 10:1 change in cross-sectional area that is necessary to produce the vowel [i] owing to the right angle bend. The vowel [i] has been shown to be extremely resistant to misidentification and is an optimum signal for estimating the length of a speaker’s SVT – a factor necessary in the process of extracting the linguistic content of human speech. Speech is a critical element of human linguistic capacity because it allows us to transmit the sounds of speech at a rate that exceeds the fusion frequency of the auditory system by merging the formant frequencies that specify individual sounds into larger syllable-sized units. The process by which we derive the speech sounds (phonemes) that convey words entails knowing the length of a speaker’s SVT. At birth humans have tongues that rest almost entirely within the mouth, similar to the tongues of apes and monkeys. Non-human tongues inherently cannot form the abrupt midpoint cross-sectional area changes necessary to produce quantal speech sounds. In the first two years of life the human skull restructures; the roof of the mouth (hard palate) moves backwards relative to the base of the skull and the tongue begins to move down into the neck. The tongue continues to move downwards into the neck, which also lengthens until it reaches adult-like oral (SVTh) and pharyngeal (SVTv) proportions. As the tongue descends into the throat, it carries the larynx down. The low human laryngeal position makes it impossible to simultaneously breathe and ingest fluids and increases the possibility of choking on food lodged in the larynx. In itself, a low larynx does not yield the capacity to produce quantal speech sounds. Many species have low larynges or larynges that can transiently move downwards. However, their tongues are positioned in their mouths and they are incapable of producing the abrupt midpoint cross-sectional area-function discontinuities necessary to produce quantal speech sounds. Archaic hominids such as the Neanderthals who had long mouths and short necks do not appear to have had human tongues. However, they most likely communicated using speech. The human tongue thus appears to be an example of Darwinian Natural Selection, enhancing speech communication at the expense of swallowing.
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