I think he's chasing dust in a sunbeam ;~D
I had a cat many years ago named Berlin (got him when the Berlin wall fell) and he used to enjoy chasing a flashlight beam on the ground. It was hysterical.
He was a real dopehead too...Steve and I used to put a little pile of catnip on the floor and he would roll in it and lay around with a dazed look on his face...then jump up like he saw a hugh rat or something and take off chasing it.
Steve and I would just laugh...it was really really funny.