Shake off the ghosts

Hello:

"Well, except for the seizures. Pointdexter had these seizures. He was possessed. He was a demon. He would bounce off the walls, he would get up a head of steam, mouth frothing, and run straight at the wall, smack into it, shake off the ghosts, and start again." From Demonology by Rick Moody

Pointdexter is a cat with seizures. What does "shake off the ghosts" mean here?