Aphasia is an acquired language disorder subsequent to brain damage in the left hemisphere. It is characterized by diminished abilities to produce and understand both spoken and written language compared with the speaker’s presumed ability pre-cerebral damage. The type and severity of the aphasia depends not only on the location and extent of the cerebral damage but also the effect the lesion has on connecting areas of the brain. Type and severity of aphasia is diagnosed in comparison with assumed normal adult language. Language changes associated with normal aging are not classed as aphasia. The diagnosis and assessment of aphasia in children, which is unusual, takes account of age norms. The most common cause of aphasia is a cerebral vascular accident (CVA) commonly referred to as a stroke, but brain damage following traumatic head injury such as road accidents or gunshot wounds can also cause aphasia. Aphasia following such traumatic events is non-progressive in contrast to aphasia arising from brain tumor, some types of infection, or language disturbances in progressive conditions such as Alzheimer’s disease, where the language disturbance increases as the disease progresses. The diagnosis of primary progressive aphasia (as opposed to non-progressive aphasia, the main focus of this article) is based on the following inclusion and exclusion criteria by M. Marsel Mesulam, in 2001. Inclusion criteria are as follows: Difficulty with language that interferes with activities of daily living and aphasia is the most prominent symptom. Exclusion criteria are as follows: Other non-degenerative disease or medical disorder, psychiatric diagnosis, episodic memory, visual memory, and visuo-perceptual impairment, and, finally, initial behavioral disturbance. Aphasia involves one or more of the building blocks of language, phonemes, morphology, lexis, syntax, and semantics; and the deficits occur in various clusters or patterns across the spectrum. The degree of impairment varies across modalities, with written language often, but not always, more affected than spoken language. In some cases, understanding of language is relatively preserved, in others both production and understanding are affected. In addition to varied degrees of impairment in spoken and written language, any or more than one component of language can be affected. At the most severe end of the spectrum, a person with aphasia may be unable to communicate by either speech or writing and may be able to understand virtually nothing or only very limited social greetings. At the least severe end of the spectrum, the aphasic speaker may experience occasional word finding difficulties, often difficulties involving nouns; but unlike difficulties in recalling proper nouns in normal aging, word retrieval problems in mild aphasia includes other word classes. Descriptions of different clusters of language deficits have led to the notion of syndromes. Despite great variations in the condition, patterns of language deficits associated with different areas of brain damage have been influential in understanding language-brain relationships. Increasing sophistication in language assessment and neurological investigations are contributing to a greater, yet still incomplete understanding of language-brain relationships.
Susan Edwards and Christos Salis
D. H. Whalen
The Motor Theory of Speech Perception is a proposed explanation of the fundamental relationship between the way speech is produced and the way it is perceived. Associated primarily with the work of Liberman and colleagues, it posited the active participation of the motor system in the perception of speech. Early versions of the theory contained elements that later proved untenable, such as the expectation that the neural commands to the muscles (as seen in electromyography) would be more invariant than the acoustics. Support drawn from categorical perception (in which discrimination is quite poor within linguistic categories but excellent across boundaries) was called into question by studies showing means of improving within-category discrimination and finding similar results for nonspeech sounds and for animals perceiving speech. Evidence for motor involvement in perceptual processes nonetheless continued to accrue, and related motor theories have been proposed. Neurological and neuroimaging results have yielded a great deal of evidence consistent with variants of the theory, but they highlight the issue that there is no single “motor system,” and so different components appear in different contexts. Assigning the appropriate amount of effort to the various systems that interact to result in the perception of speech is an ongoing process, but it is clear that some of the systems will reflect the motor control of speech.