ACHOO! How can light make you sneeze?
I follow the same routine every morning. I begrudgingly roll out of bed, get ready for the day, hustle out the side door of my apartment building on my way to the bus stop, and… sneeze. Especially in sunny San Diego where even the overcast days seem bright, I step outside and am greeted with a tingling in my eyes, prickling in my nose, and not long after, a forceful sneeze. For years, I thought I must be acutely allergic to something in the environment to cause such a reaction to stepping outside. Perhaps, though, a different phenomenon is at play: the photic sneeze reflex.
Why do we sneeze?
Before diving into the specifics of photic sneezing, what do we know about sneezing in general? Sneezing, or sternutation, describes when air is quickly forced out of the nose and mouth in response to some stimulus. In most cases, this results from irritation of the lining of the nose (the nasal mucosa). Sneezing, then, serves to clear the nasal lining of any irritant, such as a speck of dust or black pepper.
A sneeze begins when the trigeminal nerve is activated. The trigeminal nerve functions to send and receive information between the face and the brain; it carries information about sensations from the face to the brain stem, as well as controlling some motor movements of the face. The ends of the nerve fibers located in the skin of the face and nasal mucosa have a variety of different structures which allow the trigeminal nerve to recognize many different sensations, including chemical irritants, like the capsaicin that makes peppers spicy, temperature, pressure, and touch . Sneeze-relevant information is transmitted along the trigeminal nerve into a part of the medulla, the very bottom of the brainstem where the brain meets the spinal cord. This activated area of the medulla has been dubbed the “sneeze center,” as electrical stimulation of this area in animals elicits sneezes  and lesion of this area by neurological disease in some human cases abolishes the ability to sneeze .
Now that the information about whatever is causing nasal irritation has entered the medulla, a sneeze response can be organized. Neurons involved in inhalation and exhalation increase their activity and a complex response is initiated – the eyes close, a sizable inhalation takes place, and the glottis (the vocal cords and the space between them) closes. Due to this closure, pressure builds up in the lungs. Once the glottis suddenly opens, the pressure is expelled as a sudden rush of air out of the lungs and through the nose and mouth . An estimated 40,000 small particles can be dislodged from the nasal mucosa with each sneeze that clocks in at over 100 miles per hour .
What is photic sneezing?
Now that we know some things that can cause a sneeze due to irritation of the nasal mucosa, what kind of sneeze is a photic sneeze?
Photic sneezing, or the photo-sternutatory reflex, was described in medical journals as early as 1954 . A photic sneeze is one that occurs not due to irritation in the nose, but in response to a sudden bright light. Though any bright light will do in some people, most photic sneezers exhibit this reflex most strongly in response to sudden direct sunlight (think: coming out of my comparatively dim apartment stairwell into the bright morning sun).
Early investigations into the phenomenon centered around the prevalence and potential genetic inheritance of this trait. First in Swedish blood donors , then in English surgery patients and US medical students , scientists concluded that photic sneezing occurs in about 25% of the population. The Swedish scientists also found that in families in which one parent displayed the photic sneezing trait, about half of their children also exhibited light-sensitive sneezing. In families in which neither of the parents had the trait, none of their children did . This led to the conclusion that photic sneezing follows autosomal dominant inheritance. This means that the gene for the trait is carried somewhere in one of our pairs of chromosomes and only one copy of the gene (from mom or dad) is needed to exhibit photic sneezing. This finding also more recently led to an extremely clever name change for the phenomenon: autosomal dominant compelling helio-ophthalmic outburst syndrome, or ACHOO.
More recently, the actual genetic mutation for the photic sneezing trait has begun to be investigated, in part by scientists at 23andMe. Using whole genome DNA sequencing data from human participants, scientists located changes in the DNA sequence of photic sneezers near a gene associated with propensity for photosensitive seizures and another affecting the structure and function of certain brainstem areas .
Photic Sneezing Theories
Given that sneezes are mostly elicited by irritation of the lining of the nose, how could light exposure cause the same effect in a portion of the population? Though there isn’t a consensus among experts on the exact cause of photic sneezing, a variety of theories exist.
Theory 1: Trigeminal cross-talk
In this theory, it is thought that activity in nearby nerves may lead to the confusion between a visual stimulus and a respiratory response. The activity of the pupil and iris of the eye, which determines how much light our eyes can take in, is controlled by two nerves: the optic nerve, which senses light, and the oculomotor nerve, which directs the constriction of the iris muscle. This theory posits that activity in the nerves of the eye are indirectly transferred to the trigeminal nerve, which causes the nose prickling sensation and a subsequent sneeze . Similar theories also postulate some communication between these sensory modalities in areas in and around the brainstem where the neural pathways encoding each come into close physical proximity [4,5,8].
Theory 2: Parasympathetic generalization
The parasympathetic branch of our nervous system includes the nerves that control some of our bodies’ automatic functions, like digestion, salivation, and lacrimation (tears in the eye). In this theory of photic sneezing, the parasympathetic activity of the oculomotor nerve that controls pupil constriction also causes other parasympathetic actions, like increased nasal mucus production, to happen at the same time. As in a typical sneeze, this irritation in the nasal mucosa activates the trigeminal nerve to cause a sneeze .
A definitive answer to the causes of photic sneezing has not yet been reached and extensive scientific studies must be performed. Interestingly, though, one more recent study of a very small group of ACHOO patients attempted to record the electrical activity of the trigeminal nerve through small electrodes placed in the nose. Electrical activity in the nose was not increased after light exposure, which suggests that the trigeminal nerve is not irritated by something happening in the nose itself (Theory 2) and instead indicates that photic sneezing relies on a mechanism outside of the nose (Theory 1) .
Brain activity changes in photic sneezing
While there are theories as to why photic sneezing may occur, they don’t address what is happening in other parts of the brain in normal sneezers and photic sneezers. How might the structure and function of certain nervous systems contribute to the propensity for a light-induced sneeze?
Using electroencephalography (EEG) to measure electrical activity across broad areas of the brain by electrodes placed on the scalp of a small group of 10 normal sneezers and 10 photic sneezers, researchers from Switzerland began to answer this question. First, they found that two areas of the visual cortex, the area of the cerebral cortex that collects and makes sense of visual information encoded by the retina in the eye, exhibit more activation in photic sneezers than control subjects when shown a checkerboard pattern. This suggests that independent of the sneezing phenomenon, photic sneezers may have more active visual cortices.
Next, the researchers asked just the photic sneezing subjects to report how strong their experience of “nose prickling” was in response to light stimuli. When the subjects reported strong nose prickling, the EEG recordings found increased activity in two other areas of the cerebral cortex as compared to mild nose prickling trials. First, they observed increased activity in the somatosensory cortex, whose role is to process touch sensation information. Additionally, the insular cortex, a brain area hypothesized to help connect sensory experience with emotional responses, became more active with more severe prickling .
This study was completed on a small group of individuals and the authors were careful to acknowledge that these brain activity changes could result from the sneeze instead of causing the sneeze. Nonetheless, it is an interesting preliminary result to help understand photic sneezes, especially as the most popular theories tend to ignore activity in these cortical brain areas in favor of the brain stem areas that contribute to reflexes.
There are still many questions science can answer about the brain mechanisms of a universally known and widespread phenomenon like sneezing. Humans, mammals, birds, and even reptiles use this protective mechanism to keep their respiratory tracts healthy and in working order. Photic sneezing, on the other hand, has a less clear and even potentially dangerous role in the lives of a quarter of the population (any other photic sneezers feel the urge to sneeze while driving out of a dark tunnel?!). Nonetheless, this interesting trait beautifully exhibits the true complexity of how signals are transmitted through our nervous system – even if they sometimes appear to get a little mixed up.
- Nishino T (2000) Physiological and pathophysiological implications of upper airway reflexes in humans. Japanese Journal of Physiology, 50:3-14
- Nonaka S, Unno T, Ohta Y, Mori S (1990) Sneeze-evoking region within the brainstem. Brain Research, 511:265-270
- Fink JN (2001) Localization of the “sneeze center.” Neurology, 56:138
- Songu M, Cingi C (2009) Sneeze reflex: facts and fiction. Therapeutic Advances in Respiratory Disease, 3(3):131-141
- Beckman L, Nordenson I (1983) Individual differences with respect to the sneezing reflex: an inherited physiological trait in man? Human Heredity, 33:390-391
- Forrester JM (1985) Sneezing on exposure to bright light as an inherited response. Human Heredity, 35:113-114
- Eriksson C, Macpherson JM, Tung JY, Hon LS, Naughton B, Saxonov S, Avey L, Wojcicki A, Pe’er I, Mountain J (2010) Web-based, participant-drive studies yield novel genetic associations for common traits. PLoS Genetics, 6(6):e1000993
- Hyden D, Arlinger S (2009) On light-induced sneezing. Eye, 23:2112-2114
- Langer N, Beeli G, Jancke L (2010) When the sun prickles your nose: An EEG study identifying neural bases of photic sneezing. PLoS ONE, 5(2):e9208
Feature image source: By James Gathany – CDC Public Health Image library ID 11162, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6701700