Migraines are believed to be a neurovascular disorder.The phenomenon known as cortical spreading depression, which is associated with the aura of migraine, has been theorized as a possible cause of migraines. In cortical spreading depression, neurological activity is initially activated, then depressed over an area of the cerebral cortex. This situation has been suggested to result in the release of inflammatory mediators leading to irritation of cranial nerve roots, most particularly the trigeminal nerve, which conveys the sensory information for the face and much of the head. This theory is, however, speculative, without any supporting evidence, and there are indeed cogent arguments against it. First, only about one third of migraineurs experience an aura, and those who do not experience aura do not have cortical spreading depression. Second, many migraineurs have a prodrome (see above), which occurs up to three days before the aura.