amnesia, loss of memory that occurs most often as a result of damage to the brain from trauma, stroke, neurological or nutritional deficiencies (e.g., vitamin B12 deficiency), alcohol or drug toxicity, or infection.
Amnesia can be classified in different ways. The most common distinction is between anterograde amnesia, in which events following the causative trauma or disease are forgotten, and retrograde amnesia, in which events preceding the causative event are forgotten. In rare instances, an individual may experience both anterograde and retrograde amnesia at the same time.
In addition, amnesia may be transient, progressive, or permanent. For example, in transient global amnesia, memory loss is complete but temporary, lasting up to 24 hours, after which memory is gradually regained. Memory loss in degenerative brain diseases, on the other hand, tends to be progressive. Complete and permanent memory loss can result from excessive alcohol or drug intake that produces a so-called blackout state, during which the brain is unable to record new memories, leaving the individual with no memory of events for the blackout period. Childhood amnesia (or infantile amnesia) is the inability of an adult to recollect events from early childhood. This form of amnesia is generally attributed to brain development and the inability to consolidate memories in early childhood. Childhood amnesia may or may not be permanent.
When amnesia can be traced to severe emotional shock, in which personal memories (e.g., identity) are affected, the condition is typically referred to as dissociative amnesia. This condition seems to represent a psychological escape from or denial of memories that might cause anxiety. The memories are not actually lost, since they can generally be recovered through psychotherapy or after the amnesic state has ended. However, dissociative amnesia may last for weeks, months, or even years, during which a person may begin an entirely new life. Such protracted reactions are characteristic of fugue states. When the person has recovered, events that occurred prior to onset can usually be remembered, but events of the fugue period are forgotten. Posthypnotic amnesia, the forgetting of most or all events that occur while under hypnosis in response to a suggestion by the hypnotist, has long been regarded as a sign of deep hypnosis.