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When dancers spin or gymnast's flip, they rely on a sixth sense. It's how they know precisely where their bodies and limbs are. Scientists have known about this sixth sense for more than a century, but they didn't realize how much we depend on it until they studied two young people with a rare genetic disorder. NPR's Jon Hamilton reports.
JON HAMILTON, BYLINE: Touch, taste, hearing, sight, smell - those are the five senses you learned about in grade school. But Carsten Bonnemann of the National Institute of Neurological Disorders and Stroke has been focusing on a sixth sense. It's called proprioception.
CARSTEN BONNEMANN: The most beautiful demonstration of proprioception in action is Simone Biles when she was doing her gymnastics routines and spinning and somersaulting through the air.
HAMILTON: Proprioception is how Biles knows where her body is in space and where hands and feet are in relation to her body. Bonnemann says he's learned a lot about proprioception from a girl named Damiana.
BONNEMANN: She's a very bubbly and happy 9-year-old.
HAMILTON: Damiana, who turned 10 recently and lives in San Diego, was born with a rare genetic disorder. It left her with a limited sense of touch and proprioception. Damiana's mom, Diana Sawyer, says doctors spent years trying to figure out why her daughter was different.
DIANA SAWYER: She was very late at doing everything. She didn't start crawling, for instance, until she was a year and a half old. And that's, you know, late.
HAMILTON: Damiana had some problems with her feet and hips, and her spine is curved. But Sawyer says those things don't explain why her daughter has so much trouble with precise movements.
SAWYER: Buttoning - we don't really get things with buttons. She doesn't like wearing jeans, for instance. She likes wearing those kind of stretchy pants.
HAMILTON: Damiana's condition might have remained a mystery if Carsten Bonnemann hadn't seen her during a clinic in San Diego. He ordered a state-of-the-art genetic analysis which turned up a mutation in a gene called PIEZO2.
BONNEMANN: But I didn't know what PIEZO2 was really.
HAMILTON: Fortunately Bonnemann found a colleague who did.
BONNEMANN: Turns out he's in the same building just a floor down from us. So I sent him an email. Look; I have this potential problem in PIEZO2. Are you interested? Can you help me?
ALEX CHESLER: And 30 seconds later I was up in his office.
HAMILTON: That's Alex Chesler from the National Center for Complementary and Integrative Health. Chesler has spent years studying PIEZO2 in mice, but he says there's a limit to what you can learn from a mouse. So last year a team of researchers invited Damiana and a similar patient to the National Institutes of Health.
It was a revelation. Chesler says trying to understand proprioception from mouse studies had been like trying to understand Beethoven by reading sheet music.
CHESLER: But when I talked to the patient, it was like going to the symphony. Like, I was asking questions, and they were just describing things to me that a mouse - as much as we can do really amazing things with mouse research, the ability to talk to a person was just absolutely extraordinary.
HAMILTON: Chesler says the research could lead to a better understanding of sensory problems, including some types of chronic pain. And he's learned a lot about how most people use their sixth sense by studying the things Damiana and the other patient can't do.
CHESLER: They've never run. They've never jumped because those kind of actions really require precise control over your limbs in space.
HAMILTON: Carsten Bonnemann says both patients have learned to compensate for their lack of body awareness mostly by watching their own limbs as they move. Bonnemann the older patient who's in college has even learned to walk pretty well as long as she can see what she's doing.
BONNEMANN: If you take away her vision, she literally crumples to the ground. Same is true for reaching. If we cover her eyes, she gets completely off target.
HAMILTON: Of course gymnasts like Simone Biles also rely on vision during a routine, but it's their sixth sense that makes a gold medal possible. The new research appears in The New England Journal of Medicine. Jon Hamilton, NPR News.
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