Speech consists of the sounds that humans produce, most often for the purpose of expressing language orally. Speech is just one mode of expressive language; other modes are writing and the production of manual signs. Very generally, language is a system of symbols that humans use to communicate. (See ‘‘Language Disorders’’ entry in this volume.)
Although most of the time speech sounds are produced in various combinations that convey meaning (and thus are being used to express language), their combinations can be meaningless (e.g., ‘‘bababa’’). The meaningfulness aspect is the purview of language. The physical production aspect is the purview of speech.
The production of speech sounds requires that head, neck, and trunk muscles work in a coordinated fashion. Speech is often described in terms of the following component processes: respiration, phonation (voicing), resonance, and articulation. The respiratory system is the power source for our ability to produce sound. In the context of speech production, the air in our lungs is exhaled via active and passive thoracic and abdominal muscle activity, until it is halted at the larynx (in the case of voiced sounds).
The larynx sits at the top of the trachea and is comprised of muscle, cartilage, and membrane. In the context of speech production, the larynx is involved in voicing. Most speech sounds are voiced, but some are not (e.g., /s/). Without voicing, speech would be whispered. Airflow from the lungs is halted by the vocal folds (cords) of the larynx because they are closely approximated at midline. Eventually, air pressure builds up below the folds and forces them apart. The air flowing through the vocal folds sets into motion their vibration and also their cyclic opening and closing. Muscles are not the only laryngeal structures involved in voicing; the arytenoid cartilages also play an important role (their rocking motion toward midline helps to achieve complete closure of the vocal folds). The voice can change in pitch and loudness, and these too are functions of the larynx.
The vast majority of speech sounds are nonnasal (e.g., /b/, /d/), but a few are nasal (e.g., /m/, /n/). During the production of nasal sounds, the airflow above the vocal folds passes through the nasal and oral cavities. The airflow passes through the oral cavity alone during the production of nonnasal sounds. Thus, speech sounds have either a nasal or an oral resonance. Nonnasal sounds are produced when the soft palate (a muscle) moves upward and backward to make contact with the pharyngeal walls to block air from escaping through the nasal port. The soft palate is relaxed and the nasal port is open during the production of nasal sounds.
The speech sound is altered further by changing the position of the following structures of the oral cavity in relation to one another: lips, tongue, teeth, and jaw. These structures are known as the articulators, and their movement is known as articulation. In the articulation of some speech sounds, the flow of air is constricted but not stopped (e.g., /f/, /z/), whereas other speech sounds are produced by stopping the air in the oral cavity (e.g., /b/, /k/).
The central nervous system (e.g., motor cortex, upper motor neurons (UMNs), basal ganglia, cerebellum) and the peripheral nervous system (e.g., cranial and spinal nerves) are involved in speech production. UMNs originate in the primary motor cortex and project to cells in the brainstem or spinal cord. Lower motor neurons (LMNs) begin in the brainstem (cranial nerves) or spinal cord (spinal nerves) and project to the muscles on the same side of the body. For the most part, cranial nerves (which innervate head and neck muscles) receive input from left and right UMNs; this bilateral innervation offers superb protection. In the event of unilateral cortical/UMN damage, speech is affected minimally. However, unilateral cranial nerve damage has more devastating consequences for speech.
Older and younger adults can guess fairly accurately the chronological age of elderly individuals by listening to them speak (Caruso, Mueller, and Xue). However, physiological age rather than chronological age may be a better predictor of who is perceived as having an ‘‘aging voice’’ (Ramig and Ringel). Respiration and phonation are most affected by the aging process. Older people may have a restricted loudness range due to reduced vital capacity. The voice of older individuals is often perceived as hoarse. The physiological correlate of hoarseness is aperiodicity of vocal fold movement, which in the older adult may be caused by physical changes in the vocal folds (e.g., atrophy, bowing) or dehydration of the vocal folds because of decreased laryngeal gland secretions. The older voice is sometimes perceived as breathy and reduced in loudness, either of which may be due to reduced vital capacity and/or incomplete valving at the level of the larynx because of tissue changes in the vocal folds (atrophy, bowing) and/or because of changes to the laryngeal cartilages (ossification and calcification). There is not much change in the pitch of the female voice with age, except with extreme old age (higher pitch). As males age, pitch rises. A higher pitch may be the result of thinning vocal folds.
Several disease processes associated with aging can negatively affect speech production. Stroke and Parkinson’s disease (PD) can result in dysarthria, a motor speech disorder characterized by weakness, slowness, reduced range of motion, or dyscoordination of any or all of the muscles of speech. Poststroke speech impairments can include imprecise consonant articulation, a breathy voice, strained-strangled phonation, and/or hypernasality. The underlying pathophysiology of the dysarthria associated with PD is muscle rigidity, with resulting speech characteristics of monopitch, monoloudness, and reduced stress. Another motor speech disorder is apraxia of speech (AOS), a deficit in the ability to program or plan the motor movements of speech. Stroke is the most common cause of AOS.
Speech-language pathologists diagnose and treat motor speech disorders in older adults. Contact the American Speech-Language-Hearing Association for more information about speech disorders (www.asha.org).
See also BRAIN; HEARING; LANGUAGE DISORDERS; PARINSONISM; STROKE.
CARUSO, A. J.; MUELLER, P. B.; and XUE, A. ‘‘The Relative Contributions of Voice and Articulation to Listener Judgements of Age and Gender: Preliminary Data and Implications.’’ Voice 3 (1994): 3–11.
DUFFY, J. R. Motor Speech Disorders. St. Louis: Mosby, 1995.
RAMIG, L. O., and RINGEL, R. L. ‘‘Effects of Physiological Aging on Selected Acoustic Characteristics of Voice.’’ Journal of Speech and Hearing Research 26 (1983): 22–30.
YORKSTON, K. M.; BEUKELMAN, D. R.; STRAND, E. A.; and BELL, K. R. Management of Motor Speech Disorders in Children and Adults, 2d ed. Austin: PRO-ED, 1999.
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