The New York Times has an interesting article about a rare and poorly-understood form of amnesia called dissociative fugue, in which some or all memories of one’s identity become temporarily inaccessible:
Last year a Westchester County lawyer – a 57-year-old husband and father of two, Boy Scout leader and churchgoer – left the garage near his office and disappeared. Six months later he was found living under a new name in a homeless shelter in Chicago, not knowing who he was or where he came from.
Library searches and contact with the Chicago police did not help the man. His true identity was uncovered through an anonymous tip to “America’s Most Wanted.” But when he was contacted by his family, he had no idea who they were.
The fugue state is one of a number of dissociative memory disorders, all of which are characterized by an interruption of, or dissociation from, fundamental aspects of one’s everyday life, such as personal identity and personal history. During the fugue state – which can last several hours or a few months – an individual forgets who they are and takes leave of his or her usual physical surroundings. In a minority of cases, the individual can assume a new identity. Often, the fugue state remains undiagnosed until the individual has emerged from it and can recall their real identity. Upon emerging from the fugue state, the individual is usually surprised to find themselves in unfamiliar surroundings.
The prevalence of dissociative fugue is about 1 in 2,000, but the condition is more prevalent in war veterans or those who have experienced natural disasters or similar traumatic events. For example, the lawyer mentioned in the quote above was a veteran of the Vietnam war, and had walked in between the twin towers of the World Trade Center just minutes before the first aircraft struck the north tower on September 11, 2001.
Unlike most forms of amnesia, which are associated with damage to specific parts of the brain (such as the hippocampus), dissociative fugue has no known physical cause. Typically, the memory loss is triggered by a traumatic life event; subsequently, the individual enters the fugue state, during which the retrieval of memories associated with the event is somehow prevented. Thus, the fugue state is psychogenic: psychological factors impinge upon the neurobiological bases of memory retrieval. The memory loss is, however, reversible; once the individual emerges from the fugue state, he or she is once again capable of retrieving the “lost” memories.