Aphasia

A condition, caused by neurological damage or disease, in which a person's previous capacity to understand or express language is impaired. The ability to speak, listen, read, or write may be affected depending on the type of aphasia involved.

In contrast to neurological problems that affect the physical ability to speak or perform other linguistic functions, aphasia involves the mental ability to manipulate speech sounds, vocabulary, grammar, and meaning. There are several different types of aphasia. Each has different symptoms and is caused by damage to a different part of the brain.

The great majority of aphasias are caused by damage to the left hemisphere of the brain, which is the dominant

Broca's aphasia results from damage to the frontal lobe of the language-dominant area of the brain. Wernicke's aphasia is caused by damage to the temporal lobe of the same language-dominant area. (Electronic Illustrations Group. Reproduced with permission.)

language hemisphere for approximately 95 percent of right-handed people and 60 to 70 percent of left-handed people. Two areas in the left hemisphere—Broca's area and Wernicke's area—and the pathways connecting them are especially important to linguistic ability, and damage to these areas is the most common cause of aphasia. Broca's area, located in the frontal lobe of the left hemisphere, is named for the 19th-century French physician Paul Broca (1824-1880), an early pioneer in the study of lateralization (the specialized functioning of the right and left sides of the brain). Aphasia resulting from damage to this area, called Broca's aphasia, is characterized by slow, labored, "telegraphic" speech, from which common grammatical function words, such as prepositions and articles, are missing ("I went doctor"). In general, however, comprehension of spoken and written language is relatively unaffected.

Wernicke's area, in the upper rear part of the left temporal lobe, is named for Carl Wernicke (1848-1905), who first described it in 1874. Aphasia associated with this area—called Wernicke's aphasia—differs dramatically from Broca's aphasia. While speech in Broca's aphasia is overly concise, in Wernicke's aphasia it is filled with an abundance of words (logorrhea), but they are words which fail to convey the speaker's meaning. Even though their pitch and rhythm sound normal, many of the words are used incorrectly or are made-up words with no meaning (aphasic jargon). Besides their speech difficulties, persons with Wernicke's aphasia also have trouble comprehending language, repeating speech, naming objects, reading, and writing. An interesting exception to their comprehension impairment is their ability to respond readily to direct commands that involve bodily movement, such as "Close your eyes."

Certain types of aphasia—called disconnection aphasias—are caused by damage to the connections of Broca's or Wernicke's areas to each other or to other parts of the brain. Conduction aphasia results from damage to the fiber bundles connecting the two language areas and is characterized by fluent but somewhat meaningless speech and an inability to repeat phrases correctly. In transcortical sensory aphasia, the connections between Wernicke's area and the rest of the brain are severed, but the area itself is left intact. Persons with this condition have trouble understanding language and expressing their thoughts but can repeat speech without any trouble. Another type of aphasia, word deafness, occurs when auditory information is prevented from reaching Wernicke's area. Persons affected by word deafness can hear sounds of all kinds and understand written language, but spoken language is incomprehensible to them, since the auditory signals cannot reach the part of the brain that decodes them.

Most types of aphasia are accompanied by some difficulty in naming objects. However, when this problem is the only symptom, the condition is called anomic aphasia. Persons with anomic aphasia can comprehend and repeat the speech of others and express themselves fairly well, although they are unable to find some of the words they need. However, they do poorly when asked to name specific objects. Anomic aphasia is caused by left hemisphere damage that does not affect either Broca's or Wernicke's area. It commonly occurs after a head injury and also in Alzheimer's disease. Global aphasia is caused by widespread damage to the dominant cerebral hemisphere, either left or right. This condition is characterized by an almost total loss of all types of verbal ability—speech, comprehension, reading, and writing.

It is possible for people suffering from aphasia following a stroke or head injury to recover some of their language abilities with the aid of a speech therapist. However, there is little chance of recovery from severe cases of aphasia.