With the sound turned off, a hearty laugh becomes a manic rictus of pain. No artist has explored this paradox in quite the same way as Shanghai-based Yue Minjun. For more than a decade he has painted and sculpted the same laughing figure- a self-portrait repeated ad infinitum. The effect is to create an instantly-alienating world of mindless, conformist ''joy'', ironically commenting on government propaganda. So far, the Chinese government has allowed Minjun relative leeway, at odds with their censorious reputation. It probably doesn't hurt that his prices and reputation have soared in recent years, along with a rising Chinese art scene. His painting Execution sold for £2.9 million, the highest price ever paid for a Chinese work of art.