Brain Dead?

As reported by Sam Kashner in his Beat Generation memoir, When I Was Cool: My Life at the Jack Kerouac School (NY: HarperCollins, 2003), p. 43—

"I [Billy Burroughs] would ask him [his father, novelist William S. Burroughs] what he was thinking and he would tell me that he was trying to imagine what was going through Lincoln's brain when he was shot."

"A bullet?" I said.