However, the exact mechanisms have still not yet been identified.
VIA SCIENTIFIC AMERICAN:
….neurology steps in: Most experts now agree that crossed wires in the brain are probably responsible for the photic sneeze reflex…
A sneeze is usually triggered by an irritation in the nose, which is sensed by the trigeminal nerve, a cranial nerve responsible for facial sensation and motor control. This nerve is in close proximity to the optic nerve, which senses, for example, a sudden flood of light entering the retina. As the optic nerve fires to signal the brain to constrict the pupils, the theory goes, some of the electrical signal is sensed by the trigeminal nerve and mistaken by the brain as an irritant in the nose. Hence, a sneeze. But because this harmless (albeit potentially embarrassing) phenomenon doesn’t seem to be linked with any other medical condition, scientific study of the subject has been scarce. Research has done little more than document its existence and attempt to gauge its prevalence. No rigorous studies exist, but informal surveys peg 10 to 35 percent of the population as photic sneezers. A study in the 1960s showed that the trait is autosomal-dominant—the gene is neither on the X nor Y chromosome and only one copy of the gene has to be present for the trait to be expressed—so if one parent sneezes when they look at a bright light, about half of his or her children will, too. The genetic culprit remains unidentified, but scientists are starting to take an interest in trying to find out. “I think it’s worth doing,” says Louis Ptácek, a neurologist at the University of California, San Francisco, and an investigator at the Howard Hughes Medical Institute. Ptácek studies episodic disorders such as epilepsy and migraine headaches, and he believes that investigating the photic sneeze reflex could shed light on their related neurology.