Aphasiology

Aphasiology is the study of language impairment usually resulting from brain damage, due to neurovascular accident—hemorrhage, stroke—or associated with a variety of neurodegenerative diseases, including different types of dementia. It is also the name of a scientific journal covering the area. These specific language deficits, termed aphasias, may be defined as impairments of language production or comprehension that cannot be attributed to trivial causes such as deafness or oral paralysis. A number of aphasias have been described, but two are best known: expressive aphasia (Broca's aphasia) and receptive aphasia (Wernicke's or sensory aphasia).

Acute aphasias are often the result of tissue damage following a stroke.

First described by the French neurologist Paul Broca in the nineteenth century, expressive aphasia causes the speech of those afflicted to display a considerable vocabulary but to show grammatical deficits. It is characterized by a halting speech consisting mainly of content words, i.e. nouns and verbs, and, at least in English, distinctly lacking small grammatical function words such as articles and prepositions. This observation gave rise to the terms telegraphic speech and, more recently, agrammatism. The extent to which expressive aphasics retain knowledge of grammar is a matter of considerable controversy. Nonetheless, because their comprehension of spoken language is mostly preserved, and because their speech is usually good enough to get their point across, the agrammatic nature of their speech suggests that the disorder chiefly involves the expressive mechanisms of language that turn thoughts into well-formed sentences.

The view of expressive aphasia as an expressive disorder is supported by its frequent co-occurrence with facial motor difficulties, and its anatomical localization. Although expressive aphasia may be caused by brain damage to many regions, it is most commonly associated with the inferior frontal gyrus, a region that overlaps with motor cortex controlling the mouth and tongue, extending into the periventricular white matter. Not surprisingly, this region has come to be known as "Broca's area". However, an intriguing line of research has demonstrated specific comprehension deficits in expressive aphasics as well. These deficits generally involve sentences that are grammatical, but atypical in their word order. The simplest example is sentences in the passive voice, such as "The boy was chased by the girl." Expressive aphasics may have quite a hard time realizing that the girl is doing the chasing, but they do much better with "The mouse was chased by the cat," where world knowledge constraints contribute to the correct interpretation. However, "The cat was chased by the mouse" would likewise be incomprehensible. This evidence suggests that grammatical competence may be a specific function of Broca's area.

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