There are countless possible causes of headaches. “I find, almost as soon as I am recognizing the trigger in my midst — an artificial scent or bright light or barometric pressure — it’s too late and I have to ride the wave,” Cara Levine, an artist in Northern California, told me last year. Crippling, psychedelic migraines like Ms. Levine’s are often hereditary, and she recently discovered that her grandmother also apparently suffered from them.