speech

speech involves voluntary initiation and involvement of a complex set of muscles around the larynx, throat, and mouth, together with interruption of the rhythm of breathing and utilization of the muscles of expiration. Like other patterns of voluntary movement, speech originates in the cerebral cortex. Several other parts of the brain (notably the cerebellum), together with sensory feedback, modify and regulate the outgoing nerve impulses to the motor neurons whose axons activate the relevant muscles. In this instance, the motor neurons concerned are in the brain stem, and their axons travel in the lowermost cranial nerves to the muscles of the vocal apparatus. Effective speech depends also on the motor neurons in the cervical and thoracic parts of the spinal cord that serve the muscles of breathing.

The process of speech production, speech transmission, and speech perception is often referred to as the speech chain. It is the configuration of the human vocal tract that gives rise to the acoustic properties of speech. The major speech articulators are the lips, jaw, the body, tip and velum of the tongue, and the hyoid bone position (which sets larynx height and pharynx width). The configuration of the speech articulators and their co-ordinated movement generate the acoustic consequences that we perceive as the sounds of our language. These phonemes (the consonant and vowel units of language) are not produced in a sequential and isolated manner but rather are co-articulated, and they coalesce to form a complex sound stream. The speech production system may be thought of as a set of physical acoustic sources (e.g. larynx) and physiologically-determined filters (e.g. lips) that are combined. The human speech system is particularly well suited for the rapid transfer of information.

There are a variety of ways to produce speech sounds. One method involves using the air pressure provided by the lungs to cause the vocal folds of the larynx to vibrate. The resulting sound can be altered by a variety of constrictions or closures in parts of the upper vocal tract. The modern study of the physiology of speech production began in 1928 with Stetson, who measured the speed and force of articulators. The development of X-ray photography led to the dynamic visualization of the vocal tract during speech production. The sound spectrograph (developed by Koenig in the 1940s) made it possible to study speech acoustic events in greater detail and revealed phoneme-specific information in the acoustic patterns. In particular, vowel formants and consonant-dependent formant transitions were recognized as key components to phoneme identity, leading to the initial attempts at computer speech synthesis.

The acoustic properties vary among different speakers producing the same sound and, more crucially, each utterance produced by an individual is unique. The mapping between the variable acoustic characteristics of speech production and the successful and stable identification of linguistically meaningful units in speech production is a major paradox. Recent research suggests that visual information is used to resolve acoustic difficulties in speech perception.

Marjorie Lorch

See also jaw; language; larynx; lip-reading; tongue; voice.