The tendency to sneeze when suddenly exposed to sunlight or other bright light may be caused by an extra-sensitive visual cortex, the part of the brain that receives visual signals, according to a study in PLoS ONE. The so-called "photic sneeze reflex" or ACHOO (Autosomal Cholinergic Helio-Ophthalmic Outburst) syndrome affects an estimated one in four people. The study, which compared the reactions of 10 photic sneezers and 10 control subjects, appears to be the first to examine the condition. The researchers measured the electrical activity of the participants' brains while they were shown a shifting pattern of white and black squares. At around 60 milliseconds after each shift and again 140 milliseconds later, the visual cortices of sneeze-prone subjects showed significantly higher activity than those of the control subjects.