lains how listeners adjust to such differences as speech accent, voice quality, and rate of speech. Passive Listening may not be possibly hampered by an intermediate articulatory step in speech perception, but it often underestimates the variability of the pitch between acoustic signals and linguistic units (Crystal, 1987, pp. 142-8). From a teaching point of view, both theoretical approaches ought to be considered, while focusing on both speech perception and listening behavior--both or either of which may be at fault in the creation of learning problems.
Speech comprehension is differentiated from speech perception. Researchers have hypothesized that children's perceptual apparatus is "programmed" to discriminate speech sounds through "feature detectors" that respond to the acoustic and phonetic properties of speech. Speech comprehension seems to precede speech production--witness children's normal communicative development.
Disorders of cerebral hemispheres and higher brain functions
Aphasia is a "defect or loss of language function, in which the comprehension or expression of words (or nonverbal equivalents of words) is impaired as a result of injury to the language centers in the cerebral cortex" (The Merck Manual, 1987, p. 1328). Sensory aphasias are categorized into several types. In Wernicke's aphasia, normal words are spoken fluently but with neither semantic meaning nor recognition of the person's own word salad. Broca's aphasia (also known as expressive or motor aphasia)
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