An Intro to Photic Sneeze Reflex

3. What Causes Sneezing With PSR?

While the exact cause of photic sneeze reflex is unknown, there is a running theory. Scientists believe PSR has to do with the trigeminal nerve, also known as the fifth cranial nerve. The most complex of any paired nerve in the head, the trigeminal nerve has three major branches which lead to the eyes, jaw, and nose. Medical researchers surmise that signals might get crossed because the nerves are so closely clustered together. The pupils contract in bright light due to a signal that is carried along the trigeminal nerve, which also branches off to the nose. Confusion in this region could cause violent sneezing.