Imagine suddenly, and uncontrollably, having a completely different accent. You've had a stroke, or surgery, or been in a traffic accident, and when you regain your ability to speak, you sound like you're from somewhere else. Perhaps you're from Arizona and now you sound like you're from England. Or you're from Australia and sound like you're from France. Or you're from England and you sound like you're from China or ... Italy? Poland?

Foreign accent syndrome (FAS for short) is a real thing, though it's very rare — fewer than 200 cases diagnosed since it was first described in 1907. It may sound like it's just a delusion or fantasy, but fewer than 10 percent of cases have a psychological basis (for example, related to schizophrenia). Nearly all of the rest are of neurological origin: They are caused by damage or impairment to a specific area of the brain. People who have foreign accent syndrome don't think they're from somewhere else, and their language comprehension and overall verbal skills often aren't affected at all ... except for this one thing. And nearly all of them say they'd really like to sound like they used to, but though they try, what comes out sounds like it's from somewhere else. The reason for this can tell us quite a lot about how we speak, and how we produce and identify different accents.

People with foreign accent syndrome don't all have the same accent; it varies widely from person to person, and there has been at least one case of a person having several bouts of it and having a different accent each time. And while the accent may sound like it's from somewhere in particular, if you get people from that place to listen to it, they will say it's not exactly from there.

The short explanation is that FAS (possibly excepting the few psychological cases) is a disorder of speech planning and execution. It's a little reminiscent of a home-made robot that doesn't quite go where it's supposed to at the right speed and in the right way. But unlike the robot, it doesn't make a total mess; it just does it differently, in a way that seems recognizable.

That may be hard to believe. Usually if someone has a stroke and it affects their speech, they will have some kind of aphasia, which means they either can't speak at all or can't manage to speak completely coherently. Their speech is obviously broken. Foreign accent syndrome seems more like dropping a bowl and, instead of breaking, it just becomes a different style of bowl. That's because FAS is not breaking something so much as just distorting the signal from it. It's like how your voice sounds much higher for a short while after you inhale helium from a balloon. Only it's more subtle and multi-dimensional.

When you speak, you're not just saying a simple sequence of sounds. Each sound, plus the way they're put together, is a combination of several different features, and each one of those features is separately adjustable — and separately processed by the hearer. Each consonant is said in a particular place in the mouth (for example, with the tongue tip at the ridge behind the teeth), with a particular way of saying it (the difference between "d," "n," "z," and "j"), and a particular voicing ("d" versus "t," or "z" versus "s"). There are far more combinations of these three features available than any language uses, and we learn to recognize the sets different languages use. Many languages don't have the "th" sounds English has, for example, and where and how you say "r" varies quite a lot between English, French, and Dutch — and between different regional accents of English, for that matter.

Likewise, every vowel you say is made with a narrowing between the tongue and the palate in a specific area of the mouth — "beet" has it high up in front, "bought" low down in the back, "boot" high in the back. Any language divides the mouth into spaces for the different vowels with a target area for each vowel that may change depending on context (you may say the "a" in "bag" slightly closer to "e" the one in "bat," for example). Different languages and accents divide it up differently: more or fewer vowels, different targets and different boundaries. For example, Australians say "I" similar to the way Americans say "oy," and Spanish doesn't distinguish between the sounds in "bit" and "beat."

And when you say vowels and consonants in words, you say them in syllables with a specific kind of rhythm and intonation, and that, too, differs from language to language. You could say that the "music" of one language is different from that of another; it's something that can often tell you at a distance what language you're hearing, before you even make out the words.

So speech isn't really one signal, even though we receive it as one stream of sound; our mind recognizes many different things going on, just as it does when listening to a musical ensemble. And you can change a lot of things that a musical ensemble does without making it not music. Slow down Nirvana's "Smells Like Teen Spirit" and sing it in unison and it can sound like Gregorian chant — though not to a scholar of medieval music. The Beatles added a few instruments from India and it sounded "Indian" to a lot of people — though it was nothing like music from India.

In the last few decades, some interesting research into FAS has been driven by a comparatively small set of researchers. Some of the most noteworthy have been Sheila Blumstein at Brown University, Jo Verhoeven at City University London, Peter Mariën at the University of Antwerp, Stefanie Keulen at Vrije Universiteit Brussel, Barbara Tomasino at the University of Udine, Nick Miller at Newcastle University, and Jack Ryalls at the University of Central Florida. They have published a number of papers in academic journals, and Miller and Ryalls have compiled a book of the experiences of people with FAS.

The research has helped confirm that what linguists think of as different aspects of speech really are different aspects — and it has helped inform our understanding of what part of the brain controls what. Although FAS has long been associated with lesions in a left front part of the brain that is important in speech production, Verhoeven and Mariën have suggested that there may be a significant role for the cerebellum, which is important in planning and coordinating movements.

FAS helps us to see that the damage to the brain can localize so much it appears to affect just specific aspects of speech sounds, and it affects them in ways that resemble the influences or restrictions caused by speaking with a different accent. The space in the mouth used for vowels might get smaller or might shift, as it can from one accent to another. Consonants might be said more or less strongly (such as always making a "t" crisp and clear), in a different place (as with using a different kind of "r"), with a different kind of sound (such as "t" in place of "th"), with different voicing (like "z" for "s"), or some combination of those features. The rhythm of the speech might go from having an even length of time for each syllable (as in French, Welsh, or Chinese) to having an even length of time between stressed syllables (as in English, Dutch, or Russian). It might go from a fairly level intonation (as in much of the U.S.) to having considerable variation in pitch (as in much of England).

Naturally, when we hear speech tweaked on one or several of those parameters, we associate it with an accent that has a similar feature set. And if we hear an accent that sounds somehow different but we're not sure, we make our best guess at what it is. The less familiar we are with an accent, the less we're likely to notice things that aren't quite right about it: Many Americans can't tell Australian accents from English ones even though people from England and Australia have no trouble telling the two apart. Researchers studying cases of FAS have sometimes tested samples of the speech with listeners who didn't know its origin to see if they could identify where it was from, and, unsurprisingly, opinions varied considerably. But when they tested "controls" — samples of native speakers with unimpaired speech from several countries — listeners couldn't always identify their accents accurately either.

Is it really so bad for people with FAS to sound like someone else? Yes, as it turns out, it is. Our accents are very much part of our identity — they signal who we are and where we're from not only to our social groups but to that person looking at us in the mirror. And there's often considerable social stigma associated with a "foreign" accent, especially depending on where it's from. A person from Texas who sounds like Mary Poppins might just seem like she's putting on airs, but a person living in England who sounds like she's from Poland is going to get a cooler reception from some people and will find she's often being treated as though she can't understand English (which she perfectly well can — FAS does not affect speech comprehension).

For some people, FAS gradually resolves as their brain learns to adjust the signals it sends and they change their speech habits to compensate. For some, it even resolves suddenly. For others, it becomes part of their identity — for better or worse. For all of us, it helps us better understand how we produce and identify speech sounds and accents.