These specific 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: Broca's aphasia and Wernicke's aphasia. Most commonly, aphasias are the result of tissue damage following a stroke.
Broca's aphasia, first described by the French neurologist Paul Broca in the nineteenth century, the speech of its suffers have a considerable vocabulary but is lacking even basic grammar. 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 Broca's 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 Broca's aphasia as an expressive disorder is supported by its frequent co-occurrence with facial motor difficulties, and its anatomical localization. Although brain damage to many regions may cause it, it is most commonly associated with the inferior frontal gyrus, a region that overlaps with motor cortex controlling the mouth and tongue. 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 Broca's 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." Broca's 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.
Such a view is supported by the complementary nature of Wernicke's aphasia, described by the German neurologist Karl Wernicke, a contemporary of Broca. Wernicke's aphasics produce speech that seems fluent and grammatical, but is largely devoid of sensible content. Comprehension is severely impaired. Wernicke's aphasia is associated with the posterior superior temporal gyrus, known as "Wernicke's area", an area adjacent to cortex responsible for auditory processing. Therefore, the localization of the two best-known aphasias mirrors the grossest dichotomy in brain organization: anterior areas are specialized for motor output, and posterior areas for sensory processing.
A fascinating corollary of this has come from research on aphasias in deaf users of sign language, who can show deficits in signing and comprehension analogous to Broca's and Wernicke's aphasias in hearing populations. These studies demonstrate that the grammatical functions of Broca's area and the semantic functions of Wernicke's area are indeed deep, abstract properties of the language system independent of its modality of expression.