Still on bright lights and sneezes

I was wrong. Apparently you sneeze in reaction to the light, not in reaction to your pupil contracting. This is known as the photic sneeeze reflex, or ACHOO syndrome (haha!). For a certain percentage of the poplution, the nerve pathways of the normal reflex of the eye in response to light and the sneezing reflex are crossed, so looking at a bright light causes them to sneeze. Read this guy's answer to the question: "Why do we sneeze when we look at the sun?"

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An excellent question. This is far more complicated than you might imagine. Since the detailed answer below got kind of long, I will summarize the main points up front. About 25% of people do actually sneeze when exposed to bright lights like the sun. We do not know exactly why this happens, but it might reflect a "crossing" of pathways in the brain, between the normal reflex of the eye in response to light and the sneezing reflex. There is no apparent benefit from "sun-sneezing", and it probably is nothing more than an unimportant (but annoying) holdover of evolution.

What is the mechanism by which sun-sneezing occurs? What actually makes it happen? The simplest answer is that we don’t really know. There are many theories that I won’t get into specifically, since most require a detailed understanding of brain anatomy to even describe. What I will try to do instead is to briefly explain the nature of the sneeze reflex and offer a simple understanding of how light might act as a trigger.

What is a sneeze? Well, everybody knows what a sneeze is, but try and describe it sometime! The fact is, a sneeze is a very complicated thing, involving many areas of the brain. A sneeze is a reflex triggered by sensory stimulation of the membranes in the nose, resulting in a coordinated and forceful expulsion of air through the mouth and nose. A "reflex" means that some type of stimulation of your body causes you to react in a way that is NOT under your control, in other words you do it automatically without thinking and you can’t even stop it. Your body has many reflexes - the other one important to us here is called the "pupillary light reflex". If you shine a light in your eyes, your pupils get smaller, or constrict. You should be able to see this easily in a friend using a flashlight (or in the mirror).

In the pupillary light reflex, shining a light in the eye causes nerve signals to go from the eye to the brain and then back the eye again, telling the pupil to constrict. In the usual sneeze reflex, tickling the nose causes nerve signals to go from the nose to the brain and then back out to the nose, mouth, chest muscles and everything else involved in the actual sneeze. The key point is that the nerve signals take complicated routes through the brain, but usually the pupillary light reflex and sneeze reflex take different routes. Apparently what happens in sun-sneezers is that shining a bright enough light in the eye ALSO sends nerves signals from the eye to the brain and then back out to the nose, mouth and chest! In short, the wires are crossed a little bit in some people, and so shining a light in the eye "accidentally" activates two different outgoing pathways.
Tom Wilson, MD PhD