Looking
at the Sun Can Trigger a Sneeze
Have
you ever emerged from a matinee movie, squinted into the sudden burst of
sunlight and sneezed uncontrollably? Up to a third of the population will
answer this question with an emphatic "Yes!" (whereas nearly everyone
else scratches their head in confusion). Sneezing as the result of being
exposed to a bright light is known as the photic sneeze reflex. It is a genetic
quirk that is still unexplained by science, even though it has intrigued some
of history's greatest minds.
Aristotle
mused about why one sneezes more after looking at the sun in The Book of
Problems: "Why does the heat of the sun provoke sneezing?" He
surmised that the heat of the sun on the nose was probably responsible.
Some
2 ,000 years later, in the early 17th century, English philosopher Francis
Bacon neatly refuted that idea by stepping into the sun with his eyes
closed—the heat was still there, but the sneeze was not (a compact
demonstration of the fledgling scientific method). Bacon's best guess was that
the sun's light made the eyes water, and then that moisture ("braine
humour," literally) seeped into and irritated the nose.
Humours
aside, Bacon's moisture hypothesis seemed quite reasonable until our modern
understanding of physiology made it clear that the sneeze happens too quickly
after light exposure to be the result of the comparatively sluggish tear ducts.
So 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.
Epileptic
seizures are sometimes triggered by flashing lights and migraine headaches are
often accompanied by photophobia. "If we could find a gene that causes
photic sneezing, we could study that gene and we might learn something about
the visual pathway and some of these other reflex phenomena," Ptácek says. But until he and
his colleagues find the right families for their study, the photic sneeze
reflex will remain something of a genetic novelty act, like the ability to roll
your tongue. Although a 1993 paper in the journal Military Medicine raised
concerns that light-induced sneezing might endanger fighter pilots, for whom a
split second of lost vision could be lethal in certain situations, such fear
was largely put to rest when a small study found that wearing sunglasses
eliminated the effect.
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