ABSTRACT
Introduction The first significant discoveries about the neural substrates of language came from research on aphasia over 150 years ago, and sophisticated clinical and experimental studies of aphasia continue to this day to provide important insights about how
language is implemented in the brain. What exactly is aphasia? In simple terms, it is an impairment of the ability to produce, comprehend, or repeat language that results from an acquired brain injury, such as a stroke, tumor, head injury, or progressive degenerative disease. Regarding language production, some aphasic patients have great difficulty retrieving words for objects or actions; others have trouble expressing words in phonologically correct ways; and still others struggle with the grammatical structures of words, phrases, and sentences. Regarding language comprehension, aphasic patients may also manifest a variety of deficits, including impairments of phoneme perception, word recognition, and syntactic parsing. And regarding language repetition, many aphasic patients are impaired at reproducing sentences that are said to them, and some cannot even imitate single words accurately. Analogous disorders of production, comprehension, and repetition are found in aphasic patients who use sign language rather than spoken language (see Chapter 9). It is noteworthy, however, that the definition of aphasia given above rules out language disturbances that have congenital causes, such as genetically or environmentally induced perinatal brain disorders. It also excludes the following types of neurological conditions: hearing problems that arise from injury to the earliest stages of auditory processing; articulatory deficits that reflect damage to relatively low-level mechanisms for controlling the vocal apparatus; akinetic mutism, in which the patient may be completely immobile and silent; and confusional or psychotic states, in which the patient may generate abnormal language (A.R. Damasio, 1998). Aphasia, then, applies only to impairments that directly affect the linguistic system, and that are acquired as a Aphasia An acquired language deficit due to brain injury.
