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Aphasia is a disorder caused by damage to the areas of the brain that direct the ability to speak, interpret, and understand language. Usually, aphasia is caused by a head injury, a brain tumor, a stroke, or a serious infection.

In adults, one of the most common causes of aphasia is a cerebrovascular accident—a stroke. A stroke occurs when the blood and oxygen supply to the brain is blocked, either by a clogged blood vessel (cerebral thrombosis) or a burst blood vessel (cerebral hemorrhage). When an injury or stroke interferes with the blood and oxygen supply, the brain cells cut off from oxygen die.

The areas of the brain involved in communication and language—all located on the left side of the brain—include the auditory cortex, which sorts what is heard into categories that make sense; Wernicke's area, where words and word patterns are stored; and Broca's area, which receives information from Wernicke's area and sends signals to the tongue, lips, and jaw that translate brain messages into actual speech.

Because these areas of the brain control different language skills, the communication problems that occur depend on what parts of the brain are damaged. For example, if the Broca area is injured, one may understand what is said and be able to think of an appropriate response. But because the link between thought and the physical act of speaking is damaged, one has trouble coordinating lips, tongue, and jaw to form understandable words. Damage to the Broca area may also make it difficult to communicate in writing; one knows what to write but the connection between thought and hand movement to form words on paper has been damaged.

There are several different systems for classifying aphasias. Some broad areas include Wernicke's aphasia (difficulty understanding language because words spoken cannot be matched to words stored in the brain); conduction aphasia (a break in the fibers that connect the Wernicke and Broca areas of the brain; a person understands what is said, but can not repeat it); transcortical aphasia (repetition without understanding); and global aphasia (all language abilities are impaired because all portions of the brain related to language have been damaged).

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