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Why does looking at bright light trigger sneezing in some people?

Are there any recent studies that have found a cause for this Photic sneeze reflex? The Wikipedia article only references studies pertaining to the effect, stating that the cause is unknown. The article also states that "the condition affects 18–35% of the population", which seems to be quite a large percentage.

What could have lead to the development and persistence of such a genetic trait in humans? Are there any evolutionary advantages to this?

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up vote 16 down vote accepted

You are talking about the photic sneeze reflex.

The mechanisms are not entirely understood, but it affects 18-35% of the population.

According to Wikipedia (although this passage is not sourced):

The probable cause is a congenital malfunction in nerve signals in the trigeminal nerve nuclei. The fifth cranial nerve, called the trigeminal nerve, is apparently responsible for sneezes. Research suggests that some people have an association between this nerve and the nerve that transmits visual impulses to the brain. Overstimulation of the optic nerve triggers the trigeminal nerve, and this causes the photic sneeze reflex.

And, it could be advantageous:

On the other hand, some people with the trait feel that it is advantageous. In the event that nasal discomfort occurs, but to an extent that is insufficient to induce a sneeze, intentionally seeking and finding a light source facilitates the sneezing process and is in turn a mode of relief.

Also, anecdotally, as I too photosneeze, I can see an advantage: should the atmosphere become weaker in the future, and allow more light to enter than we are accustomed to, it would discourage looking directly at the sun.

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“not entirely understood” clashes with the explanation you give just below. The mechanism is entirely understood. –  Konrad Rudolph May 26 '11 at 14:11
    
@Konrad That is just one of many possible explanations. It is not known how the specific trait causes this, and why it persists to today. –  Thomas O May 26 '11 at 14:48
    
This happens to me every day. Someone told me once that it's because the 'light rattles the nerve in your eye' –  rmx May 26 '11 at 15:22
    
@Konrad: Just because a phenomenon is "not entirely understood" doesn't mean that people don't have any ideas. The reverse is also true; in fact, I can think of very few things that are entirely understood. –  voithos May 26 '11 at 21:45
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I have this 'reflex' as well. Nothing like sneezing multiple times in a row while you're driving... –  Darwy Jun 5 '11 at 21:42
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