Organic amnesia refers to any traumatic forgetting that is produced by specific brain damage. Typically, these amnesias occur as part of brain disorders caused by tumors, strokes, head trauma, or degenerative diseases, such as Alzheimer’s disease. However, certain psychoactive drugs (drugs affecting mood or behavior) can cause amnesia, as can certain dietary deficiencies and electroconvulsive therapy for depression. Organic amnesias may be temporary or permanent. Amnesia resulting from a mild concussion or from electroconvulsive therapy is usually temporary, whereas severe head injuries may lead to permanent memory loss.
The case of the patient H.M., described earlier in this article, is an example of organic amnesia. In 1953 brain surgery for epilepsy left H.M. with dramatic anterograde amnesia, meaning he was unable to remember new information and events that occurred after his operation. Somewhat surprisingly, this severe impairment in the ability to learn new information was accompanied by no detectable impairment in his general intellectual ability or in his ability to use or understand language. H.M. also showed some retrograde amnesia, or inability to remember events before the onset of the surgery. For example, he could not recall that his favorite uncle had died three years earlier. Still, most of his general knowledge was intact, and he performed well on a test of famous faces (of people who had become famous prior to 1950).
Studies of H.M. and other amnesic patients have provided surprising insights into the workings of memory. One remarkable finding is that even though H.M. had severe anterograde amnesia, he (and other amnesic patients like him) still performed normally on tests of implicit memory. For example, H.M. could learn new motor skills, even though he would have no conscious memory of doing so. Even in dense, or severe, amnesias, not all memory abilities are impaired. For more information on implicit memory, see the Implicit Memory section of this article.
Korsakoff’s syndrome, also called Korsakoff’s psychosis, is a disorder that produces severe and often permanent amnesia. In this condition, years of chronic alcoholism and thiamine (vitamin B1) deficiency cause brain damage, particularly to the thalamus, which helps process sensory information, and to the mammillary bodies, which lie beneath the thalamus. Some patients also have damage to the cortex and cerebellum. Korsakoff’s patients show severe anterograde amnesia, or difficulty learning anything new. In addition, most suffer from retrograde amnesia ranging from mild to severe and typically cannot remember recent experiences. The condition is also associated with other intellectual deficits, such as confusion and disorientation. Korsakoff’s syndrome is named after Sergei Korsakov (Korsakoff), the Russian neurologist who first described it in the late 19th century.
Amnesia also occurs in Alzheimer’s disease, a condition in which the neurons in the brain gradually degenerate, hindering brain function. Damage to the hippocampus and frontal lobes impairs memory. Many other types of organic amnesias exist. For example, in large doses, most depressant drugs can cause acute loss of memory. With severe alcohol or marijuana intoxication, people often forget events that occurred while under influence of the drug.