The sneeze

Walk out of a dim building into noon sun and, for about one person in four, the nose votes. The photic sneeze reflex - sneezing at sudden bright light - is common enough that Aristotle wondered about it in print, in the section of the Problems where he asks why the sun, which heats the body, should tickle the nose. Twenty-three centuries later the tickle is still there, and the answer is still being assembled.

The reflex is not a habit and not an allergy. In 1985, J.M. Forrester surveyed a British sample, found the response in 25 percent of it, and then read the family trees: the pedigrees were compatible with a single autosomal dominant gene. The phenomenon was accordingly christened ACHOO - Autosomal-dominant Compelling Helio-Ophthalmic Outburst - possibly the only time the medical literature has named a syndrome after the sound it makes.

Two explanations have been on the table for decades. In the crossed-wires theory, a blast of light overloads the optic nerve and the signal spills into the neighboring trigeminal nerve, which owns the nose's itch-and-sneeze circuitry. In the generalization theory, the parasympathetic nervous system fires several of its branches together - pupils constrict, tear glands start, and the nasal mucosa, sitting on the same bus, joins in uninvited. Both are plausible. Neither has been convicted.

In 2010, a Zurich team put the question on an EEG. Ten photic sneezers and ten controls watched flashing checkerboards, and the sneezers' visual cortex turned out to run generally hotter, especially in a region called the cuneus. When the flashes got bright enough to prickle their noses, the prickle tracked activity in the insula and the secondary somatosensory cortex - the brain's map of bodily sensation. One working explanation: a loud visual cortex, amplified straight into the brain's rendering of the nose. Notably, that makes the sun sneeze less a simple brainstem reflex and more a cortical event.

The same year, 23andMe researchers ran a web-based genome-wide study across nearly ten thousand participants and found the trait's molecular shadow: two common variants, one near the gene ZEB2 and one near NR2F2, associated with sun sneezing. It remains one of the most lighthearted genome-wide hits on record, and one of the more reproducible.

Mostly the reflex is harmless, except when it is not: one analysis notes that driving out of a dark tunnel into daylight roughly triples the momentary risk of a sneeze, which at highway speed is not nothing. So a quarter of humanity carries a backup sneeze trigger installed before birth, reliable enough for Aristotle to notice and specific enough for genomics to find, and no one has yet settled what the wire is doing there. The sun, apparently, is still asking.