An Internet buddy was asking about this on a police-oriented message board. There’s a homeless guy in his neighborhood who gets arrested almost every day, or at least taken into custody for public safety reasons. He has two distinct unusual mannerisms—he plays air drums constantly, and he spontaneously starts repeatedly shouting out “shit” or “mofo.” I know I’ve read about this somewhere, but for the life of me I can’t remember what causes this. For some reason my mind seems to be locking on something involving the dopaminergic nervous system—schizophrenia, maybe? Or amphetamine abuse?
ETA: He abuses alcohol too, if that helps.