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For most people who get migraine headaches, light equals pain.

Ms. HEATHER BIRD: There's a stab of pain. It's an intensifying - sometimes, it can be to the point where you feel a wave of nausea come over you.

INSKEEP: That's Heather Bird of Rochester, New York, who has a lot in common with other migraine patients who are sensitive to light - except for one thing: She's blind.

NPR's Jon Hamilton tells us how a new study of blind patients, like Bird, has allowed researchers to finally explain how light becomes pain.

JON HAMILTON: A genetic disorder has left Heather Bird with almost no vision, even though she's just 22.

Ms. BIRD: I have very limited light perception.

HAMILTON: Bird usually can't tell whether the world is bright or dim. But she says on some days, the difference becomes obvious.

Ms. BIRD: If I have a migraine and I walk outside, even if I can't see that it's a sunny day versus an overcast day, I can go, oh, my headache just got worse. The sun must be out.

HAMILTON: That sort of sensitivity to light got the attention of Rami Burstein, a migraine researcher at Beth Israel Deaconess Medical Center in Boston. He was part of a team that studied Bird and 19 other blind people who get migraines. It turned out that the people who'd lost their eyes entirely, or who had damage to the optic nerve, were not sensitive to light.

But people like Bird were. Burstein says that suggested their eyes were still sending signals from a group of photoreceptor cells that was only discovered a few years ago.

Dr. RAMI BURSTEIN (Migraine Researcher, Beth Israel Deaconess Medical Center): These photoreceptors that make less than 1 percent of all the cells in the retina in the eye carry light signals to brain areas that have nothing to do with visual perception.

HAMILTON: They don't help people see anything. Instead, these cells probably help the brain maintain things like circadian rhythms. Burstein suspected the signals from this small group of photoreceptors might be affecting migraine symptoms in people like Bird.

To find out, his team did a series of experiments with rats. They traced the signals from these special light cells in the retina all the way to an area deep in the brain. It's an area that's not involved in vision.

Dr. BURSTEIN: It has something to do with pain and it contains, specifically, neurons that are activated during migraine headache.

HAMILTON: And that would explain how light could intensify the pain of a migraine even in someone who is blind. Burstein says the finding won't help doctors treat headaches directly, but it could lead to a drug that would let patients leave the confines of a dark room. And Burstein says that would be a big deal for all migraine sufferers.

Dr. BURSTEIN: It is, for many patients, difference between reading or not reading or writing or not writing, working or not working - a lot of the things that keep their life normal.

HAMILTON: Other researchers say finding these migraine cells deep in the brain may help explain how a wide range of sensory factors can increase migraine pain. Richard Lipton directs the Montefiore Headache Center at the Albert Einstein College of Medicine in New York.

Mr. RICHARD LIPTON (Director, Montefiore Headache Center, Albert Einstein College of Medicine): Some people are unusually sensitive to smells. They can't stand the smell of gasoline or perfume. Other people may be unusually sensitive to sound. Some people are unusually sensitive to movement.

HAMILTON: Lipton says the new study won't necessarily lead to treatments for these people, but he says it should reassure them that they're not imagining things.

Mr. LIPTON: Knowing that unusual experiences in migraine have been established in our biological basis, I think is a great comfort to people.

HAMILTON: The new research appears in the online edition of the journal Nature Neuroscience.

Jon Hamilton, NPR News.

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