Ty Segall ended a chaotic performance with an admonition last night on the Late Show With David Letterman: "Go vote!" he howled. The rocker's instructions came after he and his band careened through the unhinged song "You're the Doctor," from his new album Twins. The performance included some serious headbanging, and Segall pressing his guitar against his amplifier for a little extra feedback. Letterman seemed amused by the display, saying afterward, "Remember kids, don't neglect your studies."