This is going well. Having written two posts, one has been recycled material and the second doesn’t even fall into the remit of human evolution, although it is an issue which is close to my heart: the issue of sneezing in response to sunlight or any sudden change in light levels.
Now, a good number of you will already have no idea what I’m talking about, but I hope to change that soon enough.
It’s been estimated that about one person in every three or four experiences sneezing in response to light. For me it most often happens when leaving a building or a shady area on a sunny day. Annoyingly for anyone who ‘suffers’ in this way and has looked into the condition in order to try and explain it to non-believers, it has been christened ‘ACHOO’ (for Autosomal-dominant Compulsive Helio-Ophthalmic Outburst, or minor variations thereof).
When I tell people this they usually undergo some sort of compulsive outburst of their own, except they seem to be laughing instead of sneezing.
So lets break that down a bit: studies have noticed that it occurs within families and therefore must have a genetic component (autosomal-dominant); we all know that a sneeze in not something you choose to do (compulsive); that it is related to light, in this case the sun (helio) which we detect using our eyes (ophthalmic); and sneezes result in air, spit, mucus and germs leaving our bodies (the outburst). Lovely.
Fortunately it’s also known as the Photic Sneeze Reflex (PSR) where photic refers to light, which sounds a lot more believable so I’ll use PSR from now on. Maybe you other PSR people could too, and save yourself from being the butt of a running joke. (Running joke… Running nose! LOL! Hilarious!)
Time for the interesting bit!
Setting off the Sneeze
PSR was discussed as far back as 1954 in the Journal of Neuro-Ophthalmology under the title Photosternutory Reflex. Over the years, three main ideas have been developed to explain how light entering our eyes could bring on a sneeze (reference ).
i: ‘Short Circuit’
Light entering our eyes hits the retina and causes an electrical signal to be sent to our brain via the optic nerve. This impulse may cause a similar electrical signal in the nearby trigeminal nerve which deals with facial sensations, and may mimic an irritation of the nose, leading to a sneeze. Such ‘short circuiting’ may occur because the two nerves are poorly insulated or they may run too close to one another, either of which may result from a slight genetic change from normal which is inherited from parent to child (the autosomal-dominant part).
This theory also says that the short circuit takes place at the top of the spinal cord in the brainstem – the ‘unconscious’ part of our brain which controls things such as breathing and heart rate – so we would find it difficult not to sneeze (hence ‘reflex’ or ‘compulsive’).
This is based on the knowledge that light hitting our retina also triggers our pupils to contract and our eyes to water slightly. These actions are not controlled by the same part of our brain which forms the image that we see when light hits our retina. If the light were particularly strong this signal might spill over into yet more parts of the brain which could cause us to feel a nasal irritation and produce a sneeze. This is supported by research that shows wearing the wrong type of glasses for reading can affect digestion and urination, demonstrating cross-talk between the optic nerve and the vagus nerve which runs into our abdomen.
Crucially, this cross-talk is thought to take place in the conscious part of the brain, the forebrain, meaning we might have some control over it. Which is quite likely the case, as anyone who’s tried to fight a sneeze and won will know.
This works on the same basis as Co-activation, but instead says that it is an overly-active nerve response that causes PSR, and not just particularly strong bursts of light. In other words, people like me may have jittery neurons.
A number of studies into PSR have been carried out, but until recently, no-one had managed to visualise the brain regions involved in the process. That changed in 2010 when Nicolas Langer and colleagues at the University of Zurich conducted this freely available study (reference ) using electrodes to measure brain activity of volunteers with and without PSR.
The experiments involved either showing the volunteers a black and white checkerboard pattern, or exposing them to flashes of lights. the checkerboard is the ‘control’ option as it isn’t bright and therefore shouldn’t cause sneezing.
When comparing the brain activity of the two sets of volunteers the researchers found that certain areas (highlighted in red in the images below) were consistently more active in PSR brains than in the general population. Importantly, these 4 areas were in the forebrain, ruling out the ‘short circuit’ hypothesis.
Images are taken from reference .
In the image to the left the brain areas activated are critical in visual processing, whereas those highlighted in the image below are concerned with picking up sensations from around our body. If the only areas of PSR brains that lit up were those in the image below then we could accept the CO-ACTIVATION theory. However, since areas critical for interpreting what we see are also activated when PSR brains see a boring old checkerboard pattern (left image), there is also an element of HYPERSENSITIVITY.
If you think about it, hypersensitivity must be involved otherwise everyone would have PSR if the increase in brightness was great enough.
Another finding is that the brightest flashes don’t always produce the greatest urge to sneeze, suggesting that a person’s EXPECTATION that they will sneeze also plays a part, something that can only really happen because the action takes place in the conscious part of our brain.
In short, PSR brains are slightly more sensitive even to normal visual stimuli, but there is also some cross-talk between different areas of the brain. I can personally testify to this crosstalk, as I have noticed myself sneeze on the rare occasions I pull an unruly hair from my eyebrow near the top of my nose!
But always watch this space…
As with any question, new research could easily turn up new knowledge and more definite answers, such as what is the exact nature of the genetic mutation which causes this in the first place? It would be great to be able to understand genetics from an angle that isn’t some form of developmental delay or progressive disability.
p.s. Since you ask, I always ACHOO twice. What about all you other ACHOOers out there?
 http://www.neurology.org/content/14/5/483 (Not freely available; see reference  of reference , below)
 Langer N, Beeli G, Jäncke L (2010) When the Sun Prickles Your Nose: An EEG Study Identifying Neural Bases of Photic Sneezing. PLoS ONE 5(2): e9208. doi:10.1371/journal.pone.0009208