Okay, so what are cats actually looking at when they freak out and zoom out of the room for no apparent reason?

You know, “something” catches your cats eye, and he snaps his head back and forth following a piece of dust, at most. Swats at it, zooms out of the room, runs back in a minute later and does the same thing ten more times. I always assumed it was just that: a tiny fleck of dust only a cat could see, but why does it ignite such a response in them? Its hilarious. I love watching Barnabus speed around the house. But I really don’t understand why he does it.