Since many of you asked, I'm posting a video of one of BB's episodes where she is after an invisible subject. She would do this anywhere, not just this specific spot, even in the dark. (She is not chasing shadow) If you watch it with the sound, you will hear how frustrated she was towards the end. It's not that she wants to do this, but she can't stop herself from having these compulsions, just like OCD in humans. Her OCD/CCD is not the same as typical Border Collie obsessions. (Like with water and balls) This is a medical condition that we are treating both with medicine and behavior modifications/management, possibly for life.
P.S. I had to stop filming because one of the worst things you can do with dogs with OCD/CCD is let them rehearse the compulsions and allow them to escalate.