I'm never quite sure what the official definition of migraine is, but for what it's worth every now and again I get nasty sick headaches that always start with a small 'twinkling light' warning in my peripheral vision, like a little arc of rainbow. The arc grows and grows until it fills my vision and I end up with a stinking headache and nausea. I've had these a few times when cycling, particularly in very cold weather. (It's one of the reasons I don't wear a helmet any more as wearing one seemed to trigger this process <ducks>.) But if I see even the tiniest warning light beginning and I take two ibuprofen, it usually stops the process completely, so I always carry ibuprofen with me. I have to be strict about stopping at once and taking the tablets with a mouthful of food though.