Spells of helplessness occurred, moments when he "remembered things"—blue light exploding in a black room, the glass eyes of a big toy bear—and when voices, a particular few words, started nagging his mind: "Oh, no! No, please! No! No! No! No! Oh, please don't, please!" and certain sounds returned—a silver dollar rolling across a floor, boot steps on hardwood stairs, and the sounds of breathing, the gasps, the hysterical inhalations of a man with a severed windpipe. (2.114)
Perry's describing having flashbacks, sudden episodes of vivid imagery of the murders. This is part of what his psychiatrist said was "dissociation." Flashbacks are a common experience for people who've been through life-threatening traumatic events in the past.