As mentioned previously, most cases of foreign accent syndrome are considered to be neurogenic in origin. In fact, until recently, it was thought that FAS was a completely neurogenic phenomenon. But specialists are now conceding that there are some cases in which no damage or impairment of the brain can be found. In these instances, FAS is often found to occur in tandem with a mental illness.
In some such cases, FAS doesn't just manifest in the form of vowel changes or shifts in speed of articulation, but also in the use of a different vocabulary. So, for instance, an American experienced an episode of FAS in which he not only had a British accent, but also used words like "bloke" instead of "friend," and "loo" instead of "toilet."
In another case, a Dutch woman with FAS developed a French accent and even spoke Dutch using French syntax and sometimes French words, sounding convincingly like a French person in the process of learning Dutch. The fact that she taught the Dutch language to French people was the obvious source for her knowledge of how such a person would sound [source: Keulen].
People with neurogenic cases of FAS don't insert foreign words or use foreign syntax. Neurogenic FAS is a problem of articulation alone. This difference points to why it's important to correctly diagnose the origin of a case of foreign accent syndrome. The treatment of a neurogenic FAS is a specialized form of speech therapy known as "accent reduction techniques" — i.e., retraining the larynx, tongue and lips to articulate the way they once did [source: UT Dallas].
This would be pointless for psychogenic FAS since the syndrome in such cases is a symptom of mental illness. Treatment of psychogenic FAS then requires treatment of the underlying illness; FAS often disappears as a patient recovers from an episode. The length of the episode and, consequently, the FAS, varies widely according to the illness and individual; however, for people whose FAS did fade away with an episode, the syndrome lasted for a few months [source: Keulen].
To make matters more complicated, it seems there's also something called a "mixed variant of FAS," in which the origin of the syndrome is considered to be a combination of psychogenic and neurogenic causes [source: Keulen].
As medical understanding and diagnostic tools advance, perhaps the differences between psychogenic and neurogenic causation will erode a bit. Some recent studies have found that certain mental illnesses can be caused by the immune system's response to infections, showing that sometimes, at least, the mind/body divide is not as pronounced as we might think [source: Velasquez-Manoff].