"Gamma Ray" to be specific. There was no MGMT or fatherly strings, just his usual live backing band and the darkly upbeat Modern Guilt standout. Beck looks like he's having a good time up there (note smile in the screen grab), or at least more animated than he was in the video for the track. Letterman seems to dig it, though surprisingly he doesn't make it over to hold guitarist Jessica Dobson's hand. Must be too preoccupied with the election.