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In mammals, the heart's primary pacemaker lies in a specialized group of cells -- but do non-mammalian vertebrates have a similar control system? To find out, Arrenberg et al. genetically engineered zebrafish to express light-sensitive proteins, and then used light beams generated with a digital micromirror device to locate and manipulate the function of the pacemaker cells in the hearts of zebrafish embryos -- capturing the action on high-speed video. In the first clip, illumination of the entire heart of a genetically modified fish triggers reversible cardiac arrest. In the second clip, illuminating a specific group of pacemaker cells in the heart's ventricle (square area in video) reverses the direction of cardiac conduction -- which likewise reverts to normal after illumination ceases. In both clips, high-pitched tones correspond to atrial contractions and low pitch tones to ventricular contractions.