The Photic Sneeze Reflex

Why some people sneeze when the sun comes out

I do. Repeatedly.

The nerves within the parasympathetic nervous system that coordinate sneezing feed into a part of our brainstem known as the medulla oblongata. A series of experiments conducted by researchers from Asahikawa Medical College, Japan, in 1990 showed this was the case for cats (Brain Research, vol 511, p 265), and it seems to be true for humans too since some people with damaged medullas lose the ability to sneeze (Neurology, vol 56, p 138).

Unfortunately, current brain imaging techniques are not sensitive enough to pin down exactly which neurons within the medulla control the sneeze response. It is within this elusive "sneeze centre" of the brain that the mystery of photic sneezing lies.

This mystery has a long history. In the 4th century BC, Aristotle asked why the heat of the sun prompts us to sneeze, whereas the heat of the fire does not. A partial answer came two millennia later, when the English natural philosopher Francis Bacon showed that his photic sneeze had nothing to do with heat at all: if he closed his eyes when going into the sun, he didn't sneeze even though the heat was still there.

I had no idea I was in such good company. Aristotle. Bacon. And it appears to be genetic

the correlation was too significant to ignore, suggesting photic sneezing is an inherited rather than acquired response to environmental conditions, as had previously been assumed. Subsequent studies have borne out that hunch, with patterns of inheritance suggesting that it is carried on a dominant gene (Birth Defects, vol 14, p 361), so anyone with just one copy would be afflicted. This is known as autosomal dominant transmission, giving scientists the unmissable opportunity to rename the condition "autosomal-dominant compelling helio-ophthalmic outburst" - ACHOO for short.

Oh dear.

But exactly what is the mechanism?

The answer to that, Bhutta suggests, might lie in another one of Everett's hypotheses: that the confusion arises in the way the medulla regulates our reflex actions. Everett originally proposed this idea to explain just photic sneezing, but Bhutta thinks it could explain all the strange sneezing conditions, since all of the triggers involve stimulation of a parasympathetic nerve response controlled by the medulla. When bright sunlight hits our eyes, our pupils contract involuntarily - a parasympathetic response. When our stomachs are full, the parasympathetic system kicks in to start our gastric juices flowing. When we think of sex, parasympathetic action stimulates blood flow to our genitals.

All these nerve responses flow to and from regions of the medulla close to where the sneeze centre is located. This suggests that far from being a neat system of discrete responses to individual stimuli, our reflex systems at their base in the medulla are often a tangled web of cross-talking nerve wires. Sometimes when bright sunlight hits our eyes, the parasympathetic system responds appropriately and our pupils constrict. But for certain people whose medullas are wired differently, sunlight triggers a different reflex response, such as a sneeze.

Which is why some people sneeze when they're full (I do), or when sexually aroused (NO COMMENT!!)

Nervous overkill is no deal breaker in the survival stakes as long as the right reflexes are also stimulated at the right time, so aberrant genes that cause confused reflexes in some individuals would have been conserved by evolution. "It's a mess," says Bhutta, "because it's never had to be anything else."

All this is just a hypothesis, not established fact, Bhutta emphasises, and is likely to remain so until we fashion better tools for studying the activity of individual nerve pathways in living humans. This is a sentiment echoed by Louis Ptácek, a neurogeneticist at the University of California, San Francisco. "People speak as if they know what the hell's going on," he says. "In reality, we don't."

There's a lot of that when it comes to our knowledge of neuro-anatomy. It fits though.