Wearing your emotions on your sleeve is now an idiom you can explore in real life via a biosensor wristband made by Studio XO, a London startup that turns technology into fashion statements. The pulse-tracking bracelets debuted Thursday at at the Cannes Lions advertising conference in France. More than 2,000 audience members attending the Saatchi & Saatchi's "Feel the Reel" New Directors Showcase wore the high-tech mood bracelets. As the audience watched a series of artsy ads and video shorts, LED lights embedded in their wristbands changed from cool blue through red to reveal each viewer's agitation level, their data transmitted to a screen that aggregated the overall mood of the room. Anyone brought to embarrassing tears by a particularly poignant long-distance commercial can figure out the many uses — and abuses — of such a product.