TWENTY years ago this week, two young men sat in a smoky bar in Bremen, northern Germany. Neuroscientist Christof Koch and philosopher David Chalmers had spent the day lecturing at a conference about consciousness, and they still had more to say. After a few drinks, Koch suggested a wager. He bet a case of fine wine that within the next 25 years someone would discover a specific signature of consciousness in the brain. Chalmers said it wouldn't happen, and bet against. It was a bit of fun, but also an audacious gamble. Consciousness is truly mysterious. It is the essence of you - the redness of red, the feeling of being in love, the sensation of pain and all the rest of your subjective experiences, conjured up somehow by your brain. Back then, its elusive nature meant that many believed it wasn't even a valid subject for scientific investigation. Today, consciousness is a hot research area, and Koch and Chalmers are two of its most influential figures. Koch is head of the Allen