Several years ago, the Defense Advanced Research Projects Agency (DARPA) began looking into a new medical procedure called transcranial direct-current stimulation, or tDCS. To the outsider, the experiments looked more Mary Shelley than cutting-edge neurology. Scientists taped sponges laced with metal to various parts of a patient’s scalp and then hooked them up to a small 9-volt battery — just like the ones that go in smoke detectors or old transistor radios.