The two main memory disorders are amnesia and aphasia. Amnesia is a partial or total loss of memory caused by emotional trauma, disease, or brain injury (usually due to head trauma, surgical accidents, or chronic alcohol abuse). Memory loss can occur for events just prior to the amnesia-causing incident (retrograde amnesia), or for events occurring after the incident (anterograde amnesia). In severe cases of anterograde amnesia, the person may be unable to form new memories, although recall of material learned before amnesia's onset is usually unaffected. Many cases of amnesia (even severe) are temporary, so that the person recovers his or her memory.
Aphasias are a complete or partial impairment of the ability to understand or use words which are caused by lesions in the brain. There are numerous varieties of aphasia, and diagnostic classification systems are constantly being revised.
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