Fregni and his collaborators at Harvard have published more than 200 papers on tDCS. In 2005, he co-wrote a paper showing that stimulating the left prefrontal cortex while you are doing a particular task can enhance working memory, the ability to track and mentally manipulate multiple objects of attention. He has since tested its effects on migraine, chronic pain, post-stroke paralysis, Parkinson’s disease, depression, tinnitus, fibromyalgia, marijuana craving and, strangely enough, the tendency to lie (or, as the paper more delicately put it, “the modulation of untruthful responses”).

The evidence, he said, is strongest for depression. Earlier this year he published a study in JAMA Psychiatry involving 120 people suffering from major depression. They received either 50 milligrams per day of the antidepressant Zoloft, 2 milliamps of tDCS, both or a placebo. After six weeks, the mood of those treated with either Zoloft or tDCS alone improved about equally well compared with those in the placebo group. “By itself, tDCS was exactly the same as Zoloft at relieving depression. But when you combine the two, you have a synergistic effect, larger than either alone. That’s how I see the effects of tDCS, enhancing something else.”

One of the most striking examples of cognitive enhancement comes from research supported by the U.S. Air Force, showing that tDCS improves pilots’ vigilance and target detection. “The military has been looking at how to improve vigilance for the past 50 or 60 years,” said Andy McKinley, a civilian biomedical engineer who has been studying tDCS at the Air Force Research Laboratory at Wright-Patterson Air Force Base in Ohio. “At minimum we get a twofold improvement in how long a person can maintain performance. We’ve never seen that with anything else.”

But how can a minuscule amount of electricity, applied to the skull for 20 minutes or so, like jumper cables for the brain, make people think better?

I posed that question to Roy Hoshi Hamilton, director of the University of Pennsylvania’s Laboratory for Cognition and Neural Stimulation, who has done studies on the effects of tDCS on aphasia, mental flexibility and reading ability. He leaned back in his chair and smiled.

“What is a thought?” he asked. “A thought is what happens when some pattern of firing of neurons has happened in your brain. So if you have a technology that makes it ever so slightly easier for lots and lots of these neurons, these fundamental building blocks of cognition, to be active, to do their thing, then it doesn’t seem so far-fetched that such a technology, be it ever so humble, would have an effect on cognition.” He went on to explain how the effect could endure. “There’s this mantra in neuroscience, coined by Donald Hebb: Neurons that fire together wire together. So I have this tool that makes it more or less likely your neurons will fire. Now, while I’m applying the current, I’m going to have you engage in some behavior, a working-memory task, say, or attempting to name objects even though you have aphasia following a stroke, which is my area of interest. So now that network of neurons is being activated in an environment that slightly nudges it, makes it slightly easier for the neurons to fire and the behaviors to be successfully carried out. Then it’s not too far-fetched that, when that happens over and over again, during weeks of practice, those pathways will be reinforced. I agree it does seem very simple. It’s not like we’re inserting some super-high-tech nanorobots into your brain to clear up cerebral blood vessels. But it is in accord with our thinking about how brains work. And it does appear to have a wide range of effects.”