Aphasiology
Encyclopedia
Aphasiology is the study of linguistic
Linguistics
Linguistics is the scientific study of human language. Linguistics can be broadly broken into three categories or subfields of study: language form, language meaning, and language in context....

 problems resulting from brain damage
Brain damage
"Brain damage" or "brain injury" is the destruction or degeneration of brain cells. Brain injuries occur due to a wide range of internal and external factors...

. It is also the name of a scientific journal
Scientific journal
In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research. There are thousands of scientific journals in publication, and many more have been published at various points in the past...

 covering the area.

These specific deficits, termed aphasia
Aphasia
Aphasia is an impairment of language ability. This class of language disorder ranges from having difficulty remembering words to being completely unable to speak, read, or write....

s, may be defined as impairments of language production or comprehension that cannot be attributed to trivial causes such as deafness or oral paralysis
Paralysis
Paralysis is loss of muscle function for one or more muscles. Paralysis can be accompanied by a loss of feeling in the affected area if there is sensory damage as well as motor. A study conducted by the Christopher & Dana Reeve Foundation, suggests that about 1 in 50 people have been diagnosed...

. A number of aphasias have been described, but two are best known: Broca's aphasia (expressive aphasia) and Wernicke's aphasia (receptive or sensory aphasia). Most commonly, aphasias are the result of tissue damage following a stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

.

Broca's aphasia, first described by the French
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 neurologist Paul Broca
Paul Broca
Pierre Paul Broca was a French physician, surgeon, anatomist, and anthropologist. He was born in Sainte-Foy-la-Grande, Gironde. He is best known for his research on Broca's area, a region of the frontal lobe that has been named after him. Broca’s Area is responsible for articulated language...

 in the nineteenth century, causes the speech of its sufferers to have a considerable vocabulary but lack 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
English language
English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

, distinctly lacking small grammatical function words such as articles and prepositions. This observation gave rise to the terms telegraphic speech and, more recently, agrammatism
Agrammatism
Agrammatism is a form of expressive aphasia that refers to the inability to speak in a grammatically correct fashion. People with agrammatism may have telegraphic speech, a unique speech pattern with simplified formation of sentences , akin to that found in telegraph messages...

. 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
Inferior frontal gyrus
The inferior frontal gyrus is a gyrus of the frontal lobe . It is labelled gyrus frontalis inferior, its Latin name...

, a region that overlaps with motor cortex
Motor cortex
Motor cortex is a term that describes regions of the cerebral cortex involved in the planning, control, and execution of voluntary motor functions.-Anatomy of the motor cortex :The motor cortex can be divided into four main parts:...

 controlling the mouth
Mouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....

 and tongue
Tongue
The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...

. Not surprisingly, this region has come to be known as "Broca's area
Broca's area
Broca's area is a region of the hominid brain with functions linked to speech production.The production of language has been linked to the Broca’s area since Pierre Paul Broca reported impairments in two patients. They had lost the ability to speak after injury to the posterior inferior frontal...

". 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
Passive voice
Passive voice is a grammatical voice common in many of the world's languages. Passive is used in a clause whose subject expresses the theme or patient of the main verb. That is, the subject undergoes an action or has its state changed. A sentence whose theme is marked as grammatical subject is...

, 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
Karl Wernicke
Carl Wernicke was a German physician, anatomist, psychiatrist and neuropathologist. He earned his medical degree at the University of Breslau...

, 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
Wernicke's area
Wernicke's area is one of the two parts of the cerebral cortex linked since the late nineteenth century to speech . It is involved in the understanding of written and spoken language...

", 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.

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