A significant but relatively selective inability to remember. Amnesia can be characterized along two dimensions with respect to its onset: an inability to remember events that occurred after the onset of amnesia is referred to as anterograde amnesia, and a deficit in remembering events that occurred prior to the onset of amnesia is referred to as retrograde amnesia. Amnesia can be due to a variety of causes and can be classified according to whether the cause is primarily neurological or psychological in origin. Neurological amnesias are the result of brain dysfunction and can be transient or permanent. They are usually characterized by a severe anterograde amnesia and a relatively less severe retrograde amnesia. Transient amnesias are temporary memory disturbances and can range in duration from hours to months, depending on the cause and severity. They can be caused by epilepsy, head injury, and electroconvulsive therapy (most frequently used for the treatment of depression). In cases of transient global amnesia, an extensive amnesia that is usually sudden in onset and resolves within a day, the cause is still not known, although many believe that it is vascular in origin.
Permanent amnesia usually occurs following brain damage to either the diencephalons or the medial temporal lobe. Amnesia resulting from impairment to the medial temporal lobe can occur following anoxia, cerebrovascular accidents, head injury, and viral infections to the brain. The primary structures involved in the processing of memory within the medial temporal lobe are the hippocampus and the amygdala. One of the most common causes of diencephalic amnesia is Wernicke-Korsakoff syndrome, a disorder caused by a thiamine deficiency, usually related to chronic alcoholism.
Memory impairment that is not associated with brain damage is referred to as functional amnesia. Functional amnesia can be classified according to whether the amnesia is nonpathological or pathological. Nonpathological functional amnesia is a normal memory loss for events occurring during infancy and early childhood, sleep, hypnosis, and anesthesia. Pathological functional amnesia is an abnormal memory loss found in cases of functional retrograde amnesia and multiple personality. In contrast to neurological amnesia, pathological functional amnesia is usually associated with more severe retrograde than anterograde amnesia. See Brain, Memory