In the 1980s, doctors noticed that chronic-ulcer patients who received a "vagotomy" - cutting the vagus nerve that controls the stomach - lost their ulcers and ended up with an intriguing side effect: They lost weight. The vagus nerve is the main route for signals between the stomach and the brain, regulating acid production, stomach expansion and satiation. Now medical companies are working on a pair of devices that modify these signals to help people with obesity.