Aphasia is a communication disorder that affects the ability to understand language and impairs the ability to use language without any diminished intelligence.
Aphasia is caused by neurological damage or disease to one or more brain regions specialized for language, most commonly in the left hemisphere. Aphasia results in impairment of an individual's capacity to understand or express language. The most common causes of aphasia are stroke or other cerebrovascular dysfunction such as an aneurysm. Statistics published by the National Aphasia Association indicate that between 25 and 40% of stroke survivors experience some degree of aphasia. It may also result from a traumatic brain injury, infection affecting the central nervous system, dementia, or a brain tumor. The ability to speak, listen, read, or write may be affected, depending upon 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 neurocognitive ability to manipulate speech sounds, vocabulary, grammar, and meaning. There are several different types of aphasia, the most common of which are: fluent or receptive aphasia (also called Wernicke's aphasia), nonfluent or expressive aphasia (also called Broca's aphasia), global aphasia, mixed nonfluent aphasia, and anomic or amnestic aphasia.
In fluent or receptive aphasia the ability to understand the meaning of words is impaired, but the ability to produce connected speech may be largely unaffected. Individuals may use many words, but the words may not express what the speaker intends. Pitch and rhythm may seem normal, although many of the words are used incorrectly or are nonsense words with no meaning (aphasic jargon). Persons with this type of aphasia in a more severe form may mix words that are irrelevant or not real into spoken conversation to the point that their speech becomes like babble. In addition to their speech difficulties, persons with this aphasia have trouble comprehending language, repeating speech, naming objects, reading, and writing. These individuals hear sounds or see letters but are often unable to make sense of what they mean. A notable exception to the comprehension impairment is the individual's ability to respond readily to direct commands that involve bodily movement, such as “Stick out your tongue.”
In nonfluent or expressive aphasia, speech production becomes extremely slow, is labored, limited, and stilted, and lacks grammatical function. Spoken sentences contain just a few words (four or fewer). Individuals know what they wish to communicate but are unable to express themselves in either written or verbal form. Most often, comprehension of spoken and written language is relatively unaffected.
In global aphasia, the most severe form of the disorder, affected individuals are unable to understand verbal or written words and cannot speak or write. In milder forms, individuals may be able to speak a few understandable words. Global aphasia is most common immediately after the event that causes brain damage and often improves over time, depending on the extent and severity of the neurological damage. Global aphasia is caused by widespread damage to the dominant cerebral hemisphere, either left or right.
In mixed nonfluent aphasia, individuals speak very little and with great difficulty, use few words, may be able to read and write at a rudimentary level, have limited understanding of spoken speech, and do not improve much over time.
Persons with anomic aphasia have difficulty finding the appropriate words for names of persons, places, events, or things. They may be able to read and to understand spoken language but have quite significant wordfinding difficulties, causing major challenges for both speech and writing and resulting in great frustration. Persons with anomic aphasia can comprehend and repeat the speech of others and express themselves adequately, although they are unable to find some of the words they need. They have significant difficulty 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, although it may be present in Alzheimer's disease.
Another type, 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 lost, 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, called 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.
See also Brain disorders ; Broca, Pierre Paul ; Speech-language pathology ; Wernicke, Carl.
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