THE PHYSIOLOGY OF
SPEECH The speech process involves the speech centers of the brain, the respiratory center in the
brain stem, the respiratory system, the larynx, the pharynx, the nose and nasal cavities, and the structures of the mouth and related facial muscles.
Central Nervous System
The centers for recognition and production of speech are located in the left hemisphere in 90 percent of right-handed persons, 64 percent of left-handed persons, and 60 percent of ambidextrous persons. Specific brain areas have been shown to control various aspects of the process. Speech recognition and linguistic expression are located in Wernicke's area, which receives input from the visual and auditory areas. From Wernicke's area signals are sent to Broca's area, where control of vocalization is located. The coordination of the oral motor mechanism for speech is highly specialized. The process takes place at a central level, organizing the signals that must be sent to specific muscles. The signals are sent to the larynx and oral areas by means of several cranial nerves.
Respiration and Phonation
Respiration begins with the inhalation of air. The inhalation prior to speech is somewhat quicker than when taken for quiet breathing. As air flows through the glottal opening of the throat, the vocal folds are set into vibration. The vibrating folds allow air to escape through the glottis in tiny bursts that set the air in the vocal tract to vibrating, producing sound. The frequency of the vocal fold vibrations, called the "fundamental frequency," is further modified by the shape of the air space above the glottis.
The control of vocal pitch is performed by adjustments in the intrinsic laryngeal muscles. These stretch the vocal folds, making them thinner and causing them to vibrate at a higher frequency. As a result, the column of air in the vocal tract is also excited at a higher frequency. In addition, the entire vocal tract is typically shortened by raising the entire larynx. This action amplifies higher frequencies. Control of vocal intensity is performed by adjustments in the flow of air. By using the abdominal muscles to create a more forceful flow past the vocal folds, the column of air in the vocal tract is excited to a greater extent with each cycle of vibration. This increases the perceived loudness of the voice.
Resonance The resonance of a voice gives it its characteristic quality. The voices of individual speakers can be distinguished because of this quality. Because resonance is determined by the shape of the vocal tract, some of its aspects cannot be varied. However, the shape of the vocal spaces can be changed in a number of significant ways: positioning of the
tongue, movements of the jaw and lips, constricting of the pharynx, andÑmost importantlyÑraising and lowering the soft palate, which couples and uncouples the nasal cavities from the vocal tract. Alterations in resonance produced by changes in position of tongue, jaw, and lips are responsible for vowel sounds. Adding the nasal cavities to the vocal-tract space creates the characteristic resonance of nasal sounds.
Articulation
The articulators involved in speech sounds are the lips, jaw, teeth, different regions of the tongue, gum ridge, hard palate, soft palate, and glottis. The sounds made by these articulators are combined with phonation for voiced sounds and uncombined for voiceless ones. The lips may be rounded or retracted, movements that add a characteristic resonance to certain vowels. In addition, the lips may articulate with each other and with the teeth to produce several different kinds of sounds. The tongue, the most movable structure in the vocal tract, is usually considered to consist of three articulatorsÑtip, blade, and backÑthat are partially independent. The tongue can articulate with the teeth, gum ridge, hard palate, and soft palate to produce a great variety of plosive and fricative consonants. In addition, tongue positioning is therimary adjustment in the production of vowels, diphthongs, and semivowels.
The jaw moves only vertically in speech production, increasing and decreasing the size of the oral cavity so as to allow for higher or lower tongue position. The glottis, in addition to vibrating for phonation, can produce some other sounds. By abruptly stopping and restarting phonation, it also can create the glottal stop that occurs in certain dialect variations.
Hearing
The importance of hearing in speech production depends strongly on the age of the speaker. Infants born with profound hearing loss cannot learn to speak without speech therapy and amplification of their hearing. Often, even with these aids, the speech of the congenitally deaf is less than perfectly normal. Even a profound loss of hearing in both ears after language has been acquired, however, will result only in a slow drifting away from correct pronunciation. Usually the sounds produced remain intelligible to listeners.