This book talks about these types of migraines: "It's Not Mental" And I had two family members with both the migraines with pain and without pain. It is a neurological phenomenon, but did involve blood vessels as well. The sensation can be like seeing static, faded colors, falling down, not able to think a thought through, speechlessness. One friend had this and was taken to the ER thinking it was a stroke.

As for cause, it seems to be diverse ranging from hormones, to gluten issues, to B vitamin problems or CoQ10 deficiency to unknown.