I've had this for years. As Dr. Oyakawa says, it's a migraine headache. Specifically you (we) are experiencing what's called the 'aura,' a sparkly, rainbow, zig-zag effect that in some people never develops into the classic pain, though the rainbow is usually followed by the standard one-sided headache.
It's so well known that there's lots of artwork-type depictions of it. Take a look:
http://www.google.com/images?hl=en&um=1&ie=UTF-8&source=og&sa=N&tab=wi&biw=1335&bih=840&q=migraine%20aura&tbs=isch:1
i have the same problem. its happening to me right now. i don't know what it is.