Severe psychological trauma can sometimes cause functional amnesia. People with functional amnesia seem to have nothing physically wrong with their brain, even though the traumatic event presumably affects their brain in some way. In dissociative amnesia (sometimes called limited amnesia), a person loses memory of some important past experiences. For example, a person victimized by a crime may lose his or her memory for the event. Soldiers returning from battle sometimes experience similar symptoms.
Another type of functional amnesia is dissociative fugue, also referred to as functional retrograde amnesia. People with this disorder have much more extensive forgetting that may obscure their whole past. They commonly forget their personal identity and personal memories, and they often unexpectedly wander away from home. Typically the fugue state ends by itself within a few days or weeks. Often, after recovery the individual fails to remember anything that occurred during the fugue state.
Dissociative identity disorder, also called multiple personality disorder, is a type of amnesia in which a person appears to have two or more distinct personal identities. These identities alternate in their control of the individual’s conscious experiences, thoughts, and actions. In many cases, the person’s primary identity cannot recall what happened while the individual was controlled by another identity..
Although functional amnesias are a recurrent theme of television shows and movies, relatively few well-documented cases exist in the scientific literature. Most experts believe that these conditions do exist, but that they are exceedingly rare.