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A weekly show where we endeavor to answer one of your big questions. This week, ReiAyanami8 asks, "Why does sudden exposure to sunlight cause some people to sneeze?"
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Hi, I'm Craig, (sneeze) and this is Mental_Floss Video. Today I'm going to answer ReiAyanami8's big question, "Why does sudden exposure to sunlight cause some people to sneeze?" So this phenomenon has a name: photic sneeze reflex, or you can call it the autosomal dominant compelling helio-ophthalmic outburst, also known as (sneeze). Let's get started.

(Intro)

According to various studies, between 10 and 35 percent of people have the photic sneeze reflex, and it's actually genetic. One study in the 1960's revealed that if you have the photic sneeze reflex, there's a 50% chance that your child will inherit it. There haven't been too many studies on why it exists, though, and here comes the most common sentence in the Big Question: THERE ARE A FEW PREVAILING THEORIES.

One theory came from a psychiatrist named Henry Everett in the 60s. He suggested that the nervous system is to blame. It's responsible for sending signals that result in physical reflexes, and Everett believed that the photic sneeze reflex is an example of mixed-up nerve signals.

Modern-day scientists with their iPhones and their Snapchats and their Priuses have narrowed it down to two reflexes that may be the culprits: the pupillary light reflex and the sneeze reflex. The pupillary light reflex is fun to say, and it's what causes our pupils to get smaller when we enter environments with bright light. A signal gets sent from the nervous system to the optic nerve, which controls the eye. And the sneeze reflex is what causes a sneeze when the trigeminal nerve in the brain feels an itch in the nose. Normally these two reflexes are unrelated, but experts believe that their paths may cross in the people who have the photic sneeze reflex. That's possible because the trigeminal nerve and the optic nerve are close together.

Another theory is that it could be related to parasympathetic generalization. It's what happens when the parasympathetic nervous system gets stimulated by something, then the whole system activates. That could potentially cause light-induced sneezing. (pause, sneeze)

As I mentioned earlier, the photic sneeze reflex is genetic. Experts don't know the specific gene that causes it, but interestingly, consistency between families has been observed. For instance, bright light might cause someone to sneeze just once. Some people experience three to five sneezes. And the amount of sneezes tends to be similar within a family. (sneeze, sneeze, sneeze!) Thanks, mom.

The reflex is still being researched, because some neurologists believe that it might lead to more information on the causes of epilepsy and migraines. Those two conditions are sometimes set off by light as well, so they might be related to the photic sneeze reflex.

Thanks for watching Mental_Floss Video, which is made with the help of all of these autosomal dominant compelling helio-ophthalmic outbursts. If you have a big question of your own you'd like answered, leave it below in the comments. See you next...week.