A cheat sheet for the strange case of Michael Boatwright, the 61-year-old who reportedly forgot his native language.

Michael Boatwright, a Florida man found unconscious in a California Motel 6 months ago, woke up in a Palm Springs hospital in February speaking exclusively Swedish. News of Boatwright's strange memory loss has been making headlines in the past few days, garnering attention across the U.S. and in the U.K. and Sweden. The 61-year-old Navy veteran doesn't recognize himself in ID photos, has no memory of who he is, and calls himself Johan Elk, The Desert Sunreports.

According to the paper, he has been diagnosed with Transient Global Amnesia, an inability to make new memories, and as well as being in a fugue state, a kind of blackout that can cause people to wander away from home unexpectedly and create new identities.

Though details on Boatwright's specific case are still pretty hazy, here's a quick rundown of what we know about conditions like his:

What is amnesia, really?
There are two broader categories of amnesias: those caused by physical trauma in the brain, and those caused by a psychological issue.

Organic amnesia is caused by brain damage--sometimes from seizures, brain inflammation or diseases like Alzheimer's. Much of what we know about it comes from a famous patient known by the initials H.M., one of the most important research subjects in brain science. He underwent surgery to treat severe epileptic seizures in the 1950s, and due to the removal of part of the medial temporal region of his brain, exhibited profound amnesia for the rest of his life. He couldn't form any new memories, though he could remember details of the 27 years of his life before the surgery.

Organic amnesia, resulting from brain damage, don't usually make people forget who they are. "They lose their most recent memories, not their oldest ones," cognitive neurologist Barry Gordon writes in his book Memory: Remembering and Forgetting in Everyday Life. "Real amnestics rarely—if ever—forget their own names. What they cannot do is learn new names. They can hardly store any new information at all." Think Drew Barrymore in 50 First Dates, rather than Anastasia in the animated movie.

Psychogenic amnesia, on the other hand, occurs without structural damage to the brain. It can be caused by mental or emotional stress. A fugue state, another one of Boatwright's diagnoses, is a psychogenic form of amnesia.

A fugue what? 
A fugue state, sometimes referred to as dissociative fugue or psychogenic fugue, is a dissociative disorder, meaning it causes a breakdown in the normal function of memory, identity, perception, etc. I's a temporary bout of amnesia, usually lasting for a few days or perhaps weeks, that often induces someone to drop whatever they're doing and wander away from home or work. The person then will wake up in a different place, unaware of who he is or how he arrived there, as Boatwright seems to have.

The onset of a fugue state has been associated with severe stress. One case details a 28-year-old medical student in Nigeria who, faced with extreme economic and academic pressure, blacked out while studying alone in his room one night and came-to at his brother's house almost 400 miles away.

A 1999 study in Psychological reports described the fugue state as "one of the least common and certainly the least studied dissociative disorder." It's a pretty rare disorder, affecting an estimated 0.2 percent of the population, according to the International Society for the Study of Trauma and Dissociation. Because of its relation to stress, its prevalence tends to increase during natural disasters and wars.

So what is Transient Global Amnesia?
Transient Global Amnesia--which Boatwright has also been diagnosed with--is characterized by a temporary, abrupt inability to make new memories, without any other neurological disfunction clouding consciousness, perception or identity. Patients remain alert, asking a lot of repetitive questions about their situation like "what am I doing here?" then immediately forgetting the answers. About 5 in every 100,000 people are affected by TGA.

After an average of four to six hours, the ability to remember new information typically returns. It can happen only once in a person's life, though in rare cases, it recurs.

What causes it precisely is still contested, but can be triggered by either physical or psychological stresses, and usually affects people older than 60, according to a 2012 study inThe Lancet. "A great number of TGA attacks follow a series of potential precipitating factors and activities such as strenuous physical exercise, sexual intercourse, hot and cold baths or showers, emotional stress, driving or riding a motor vehicle, and medical procedures," one studynotes. So that's basically everything.

OK, but how does that explain the Swedish?
Boatwright moved to Sweden in the 1980s and learned to speak Swedish, according toAftonbladet, a Swedish newspaper that picked up the story.

"Amnesia doesn’t typically cause a loss of language," Gordon says, though brain damage from strokes can often lead to language difficulties. Similarly, a 2012 study in the Journal of the American Medical Association Neurology on three transient global amnesia patients found that there might potentially be a relationship between recurrent TGA and primary progressive aphasia, a degeneration of language and speech capacity. The patients studied struggled to find the right words and name objects correctly.

However, a few reports describe patients who, like Boatwright, could still speak well--just not in their native language.

Famously, there's the case of Anna O., a young Austrian woman whose treatment led to the rise of "talk therapy" in the 19th century. After the death of her father, she experienced a slew of psychiatric issues that caused, among other things, an inability to speak her native language. She spoke in English, and could still read French and Italian, but sometimes lost her command over her first language, German.

More recently, a study from 2004 in Neuropsychologia detailed the case of 33-year-old German man who was admitted to a psychiatric ward claiming no memory of his identity or location. F.F., as the paper calls him, spoke English with an accent and said he had no ability to either understand or speak his native language. He had disappeared from his home in Germany four months prior.

Less than a week after he was discharged from the hospital, he returned to Germany, and his father reported that he could communicate in German, though he was "somewhat dysfluent." The study noted that even as he claimed no knowledge of German, "he appeared to retain implicit knowledge of autobiographical facts and of the semantic or associative structure of the German language."

The authors noted difficulty in assessing how much language he actually lost:

[I]t is often difficult to distinguish between an amnesia that might be 'real' and one that is feigned, just as it is often impossible to be certain whether an amnesia is organic or psychogenic. It may even be the case that an apparent dissociative amnesia has both real and fabricated aspects. To be absolutely certain of any diagnosis of dissociative amnesia or fugue is thus, in most cases, extremely problematic.

As Gordon put it, "In medicine, it’s always the case that strange things can happen."