Thanks to a mouse watching clips from “The Matrix,” scientists have created the largest functional map of a brain to date. It’s a diagram of the wiring connecting 84,000 neurons as they fired off messages in a piece of that mouse’s brain about the size of a poppy seed. The project may help unravel the circuitry our brains use to work, wiring that if disrupted can lead to diseases. More than 150 researchers were involved in the work, led by scientists at the nonprofit Allen Institute, Baylor College of Medicine and Princeton. It was published Wednesday in Nature.