August 3, 2007  -- "On the 15th of March, about five in the morning, he heard an explosion like a gunshot, and woke to find himself in a strange bed in a town he did not recognize."

No, this is not the story of the fictional amnesiac Jason Bourne of "The Bourne Ultimatum," which opens this weekend.

It is the story of another Bourne: The real 19th century Ansel Bourne that most likely served as inspiration for the super spy's name.

According to "Zoar, or The Evidence for Psychical Research Concerning Survival" by William H. Salter, former president of the Society for Psychical Research, one day a man calling himself A.J. Brown arrived in Norristown, Pa., and opened a small shop.

Nearly two months after his arrival on the morning of March 15, the man who had been calling himself Brown opened his eyes and had no idea where he was. Frantic, he ran to the neighbors and was astonished to learn he was in Pennsylvania.

Even more awestruck were the neighbors, who learned that the man's name was not Brown, but Ansel Bourne and that he was an evangelical preacher who lived in Rhode Island. Ansel couldn't remember anything after he set out to visit his sister's house two months before.

Ansel's "Bourne Identity"-like case was one of the first documented instances of a rare disorder known as dissociative fugue.

The Fugue State

"[Dissociative fugue] is a state in which an individual has lost their identity," explained Daniel Schacter, a psychology professor at Harvard. "They don't know who they are, and they've lost all information about their past. They go on functioning automatically."

According to Elisabeth Moes, associate professor in the psychology department at Suffolk University, dissociative fugue can be brought on by traumatic events, "usually after a physical injury together with a lot of stress. At that point they can become amnesic."

Then the victim enters a "fugue state" in which he or she functions completely normally, even though there is no recollection of who he or she is.

Unlike the fictional Jason Bourne, who immediately tries to find his identity, victims in the fugue state don't even realize anything has gone wrong, Schacter told ABCNEWS.com. Rather, they can take on a new name and occupation, as if they've never missed a beat in their new life.

"That's typically the kind of thing you hear about," said Scott Lilienfeld, a psychology professor at Emory University. "A person disappears, and then shows up in a new area under a new identity with a new job and new friends."

According to Moes, unlike general amnesia, victims of dissociative fugue have an easier time recalling their past lives than people with Alzheimer's or victims of head injury, but there is no textbook treatment.

"You just have to find their memory," explained Moes. "Help them piece it back. Have them use imagery, anything that could trigger the recollection of events."

While the fugue state typically can last up to a few months, the condition can, if nothing triggers old memories, last a lifetime.

Once some of the past memories are triggered and the victim realizes that they aren't who they've claimed they are, Schacter believes the victim enters into a state of functioning retrograde amnesia. Like Jason Bourne, the victim can actively try to discover the rest of their past.

La Belle Indifference

Dissociative fugue is extremely rare and also very controversial in the world of clinical psychology.

"Some people are skeptical about the whole diagnosis," Moes told ABCNEWS.com. "It's easy to fake."

Lilienfeld is skeptical because of the lack of care or worry expressed by the victims. In the medical world, such a lack of emotion is known as "la belle indifference."

For Moes, it's the indifference to such a severe situation that proves it's a "bonafide neurological problem."

Lilienfeld, however, believes the indifference can mean other things.

"A cynic might say that they [dissociative fugue victims] are indifferent because they did it on purpose," said Lilienfeld, who believes people could fake the disorder to get away from financial or other troubles. "I worked in a lot of hospitals, and I've seen thousands of patients, but I've never heard of a case."