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The Speech Chain

Speech therapist examining the tongue and throat of a child

Communication takes place between two individuals, one of them is the speaker and the other is the listener. The channel that sends the communication is very important. What does the speaker do? The speaker first decides to say something (intention) to another human being (or to machine). This event takes place in the higher centers of the mind/brain.

The desired thought passes through the language centers of the brain where it is given expression in words which are assembled together in the proper order and given final phonetic, international, and durational form (language). The results of the language production centers of the brain may be considered as speech motor program which executes over time by conveying firing sequences to the lower neurological centers, which in turn impart motion to all of the muscles responsible for speech production, the diaphragm, the larynx, the tongue, the jaw, the lips, and is in, much if not all of this activity is subconscious, and involves constant corrective feedback.

Girl showing her tongue and throat to speech therapist for inspection
Girl showing her tongue and throat to speech therapist for inspection

As a result of the muscle movements , a stream of air emerges from the lungs, passed through the vocal cords where a phonation types( e.g. normal voicing, whispering, aspiration, creaky voice, or no shaping whatsoever) is developed, and receives its final shape in the vocal tract.

Before emerging from the mouth and the nose and through the tissues of the face, the vibrations caused by the vocal apparatus of the speaker get radiated through the air as a sound wave. The sound wave may be converted to analog or digital form for storage or transmission, and in the form of electric waves may be transported thousands of miles to its destination, where the information in the electric waves is converted back to the form of sound. It is in the form of an electronic copy of the original sound wave that automatic speech recognition by computer gains access to speech data.

The sound wave, which may have passed through electronic coding and decoding, eventually strikes the eardrum of another human being, where it is first converted to waves on the surface of the tympanic membrane, next to mechanical motion via the follicles of the middle ear, then to fluid pressure waves in the medium bathing the basilar membrane of the inner ear, and finally to firings in the 30,000 neural fibers which combine to form the auditory nerve (hearing). The lower centers of the brainstem, the thalamus, the auditory cortex and the language centers of the brain all cooperate in the recognition of the phonemes which convey meaning, the intonational and durational contours which provide additional information, and the vocal quality which allows the listener to recognize who is speaking and to gain insight into the speakers health emotional state, and intention in speaking (auditory and language processing)

Speech therapist training a child
Speech therapist training a child

The higher centers of the brain, both conscious and subconscious, bring all the experience of the listener to this incoming auditory and language data in the form of previous memories and understanding of all the current context, allowing the listener to manufacture in his or her mind a more or less faithful replica of the thought which was originally formulated in the speakers consciousness and to update the listeners description of the current state of the world (understanding). The listener may in turn become
the speaker, and vice versa.

In simple words, the concepts are generated (encoded) in the speaker’s brain which sends neural commands to the muscles of speech. By moving various muscles the speaker speaks. This is transferred as acoustic waveform and reaches the listener ears. The middle part of the ear converts this acoustic energy in to mechanical energy. The inner part converts the mechanical energy into electrical impulses. These impulses are carries to the brain of the listener which decodes the concept. This process, of transfer of the concept from the speaker’s brain to that of the listener, is called the speech chain.

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