Kathleen Bogart has a musical voice, with warm inflections and a ready laugh.

It's not just a reflection of her personality. It's a tool she uses to convey the emotions that her face cannot.

Like others with the rare condition known as Moebius syndrome, her face is almost completely paralyzed. She must use her tone of voice, her gestures and other adaptations to let people know what she is feeling.

Ms. Bogart is unusual because she not only has Moebius syndrome, but she does research on it as a doctoral student in psychology at Tufts University outside Boston.

In one study, she and her mentor, David Matsumoto of San Francisco State University, found that people with Moebius have a normal ability to detect emotions in others.

That might seem only logical, but some had speculated that people with facial paralysis might struggle with sensing emotions because of a concept called the "facial feedback hypothesis."

Championed by psychology pioneer William James in the late 1800s, the facial feedback hypothesis says that our emotions are actually shaped by our facial expressions.

That ought to mean that people with Moebius should have a blunted ability to feel or identify emotions, but Ms. Bogart and Dr. Matsumoto found just the opposite.

If anything, she said in a recent interview, "I think an interesting benefit of Moebius is that it may make you more attuned to other people's emotional cues so you can translate them into other ways to express those emotions back to people."

In a separate study, Ms. Bogart found that people with Moebius are fairly well-adjusted, but struggle with social interactions, particularly with strangers.

In her own case, she has been able to make a slight smile -- with a deep left dimple -- since she was about 10. While that helps her in public, "being able to mirror other emotions would be really useful," she said. "When someone's telling me a sad story, I can't mimic their expressions."

To compensate, she'll adopt a sympathetic tone and say things like, "Oh, that must have been really sad for you."

Having even a slight smile is important, she said, because smiles are the gateway to so many social interactions, from being polite to laughing at a joke to showing an interest in what someone's saying.

That is the basis for the work being done by Ronald Zuker, a plastic surgeon at the Hospital for Sick Children in Toronto.

Dr. Zuker has created the ability to smile in about 200 patients with Moebius over the past several years by transplanting pieces of their thigh muscles to both sides of their faces.

"When people with Moebius encounter somebody face to face, they don't respond as one would expect, and their response is misinterpreted as being not friendly or stupid or not involved, which is probably not true at all," he said, "but that is the impression that is given because they can't animate their faces."

His surgery can at least give them one expression, and "it's the one that is positive.

"You wouldn't want to only have a frown; it wouldn't make you the most popular guy in town," he said. "When people are smiling at you and you are able to smile back that's a sign of friendliness and being personable."

Most people with Moebius have missing or atrophied sixth and seventh cranial nerves, which power the movement of the facial muscles. But their fifth cranial nerve, which provides facial sensation and the chewing movement of the jaw, is usually intact.

That allows Dr. Zuker to attach one end of the inner thigh muscle to the corner of the patient's mouth, beneath the skin, and the other to the jaw nerve. When they clench their jaw muscles, they can smile.

By giving them control over the lower lip, the surgery also helps many Moebius patients avoid drooling and do a better job making "b" and "p" sounds when they speak.

The surgery can make a big difference in a patient's social life, Dr. Zuker said.

"I remember getting a call from a teacher of a 7-year-old little girl, and she said, 'I don't know what you did, but this little girl before your surgeries would hide under the table and was unable to interact with any of the kids in the class, and now she is talking and laughing and is one of the gang.' "

But even with a smile, people with Moebius still can struggle in social settings, especially when they are trying to convey more subtle emotional signals.

And while Moebius itself is rare -- there may be only about 2,500 cases worldwide -- other kinds of facial paralysis are more common. Bell's palsy, which often paralyzes half the face, strikes nearly 75,000 people a year in the United States, for instance.

Vicki McCarrell, president of the Moebius Syndrome Foundation and mother of a college-age son with the condition, said that when you know people with Moebius, you can pick up their emotional messages pretty easily.

She's reminded of that every time she goes to a foundation meeting.

"After a few hours, you forget you're with a group of people who can't move their faces because they truly do have these great adaptive skills using their vocal inflections and their humor and body gestures," she said.

Yet they can struggle in the outside world.

Besides his Moebius, Ms. McCarrell's son has a severe allergy to sunlight, and she once told him that she imagined that was a worse handicap.

"He said, 'No mom, you don't get it. That I can deal with, but the tough thing is I look in the mirror every morning and I cannot change my face.' I think of him as a good-looking, funny kid, but he doesn't think so."

Doug Oster writes a blog, "Growing With Doug," exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.