I've never had a migraine headache, but my wife gets them like three or four times a year, usually pieces by a visual aura (and sometimes if she just goes into a dark, quiet room, she only gets the aura). She always described it as an "electric caterpillar" crawling from the side. I was glad she had told me about it in detail because several years ago I got one, and that's the only reason I knew what was happening. Mine wasn't as colorful as what she described, but otherwise the same.

Here's a short video simulating one, and it's very similar to my experience (and, apparently, hers).