Yes, Foreign Accent Syndrome (FAS) is a genuine phenomenon. It is typically the result of brain trauma (specifically left-brain), and in the vast majority of documented cases of FAS, the sufferers are also recent stroke victims. It is usually a transient phenomenon.
FAS is actually something of a misnomer, since what actually happens is that pronunciation and vocal stress patterns change dramatically enough to be perceived as a foreign accent, although listeners may not always be able to agree on exactly what accent it is. Studies by linguists have ascertained that these changes are inconsistent within individuals, and furthermore are inconsistent with the actual foreign accents that they are said to represent.
The clearest documentation of this that I have found is the case of someone who originally spoke with a Tyneside accent (a strong regional accent from the north of England) and who, while suffering from FAS, retained many distinctive features of her Tyneside accent, but also acquired certain other features, such as an added vowel after word-final consonants. This added vowel gave listeners the impression of an Italian accent.
It should be noted that FAS does not refer to cases where an individual's accent reverts to one which they originally spoke with but lost in later life, although such cases are also often linked with brain trauma, as in the following:
"Acute stroke resulted in right hemiparesis and the inability to speak. Two months after the stroke, the man '...began to develop a nonfluent Broca's aphasia, with characteristics of speech that sounded like a Dutch accent.' .... The researchers discovered that the man was born in Groeningan, Holland, and moved to the United States at the age of five years. As a teenager he had lost his accent completely and acquired an American dialect." (1)
Also, FAS does not refer to those cases (sporadically reported but never, as far as I can ascertain, substantiated) of trauma victims suddenly speaking fluently in languages which were previously unknown to them, as in the following:
"Unfortunately, there was a rather nasty crash about five miles from the finish on the descent. The most seriously injured was Katherine Francis, who spent the night in the hospital and reportedly woke up from her concussion speaking Spanish, which she does not know." (2)
Most studies in FAS are relatively recent; indeed, as late as 1994 there were only about half a dozen cases described in clinical literature, although it was always generally accepted that the syndrome was considerably more widespread and frequent than that. The main reasons given for this lack of study are the transient nature of the syndrome, coupled with the fact that sufferers usually have other more severe problems. (3) I do wonder though if it might also be because there's an air of disrepute regarding this and other popularly reported phenomenon. Most scientists want above all to be taken seriously, and I can see how the risk to reputation involved in a study such as this might act as a strong discouraging factor.
- The FAS thread from October 1997 on http://www.emich.edu/~linguist/issues/8/8-1510.html and linked pages
- (1) "Foreign Accent Syndrome After A Stroke Is A Rare Occurrence", 30 May 1997, at http://web.missouri.edu/~psycmm/abnormal/msg00025.html
- (2) Cartalk letters, 12 September 1997 at http://cartalk.cars.com/Mail/Letters/09-12-97/5.html
- (3) An older (20 Jan 1994) FAS summary at http://www.emich.edu/~linguist/issues/5/5-69.html
- http://www.emich.edu/~linguist/issues/8/8-1510.html contains a huge list of further web and printed articles on FAS and related subjects. I also recommend reading the rest of the thread, which is fascinating.