Dozens of other studies have turned up additional evidence that brain stimulation can improve performance on specific tasks. In some cases, the gains are small—maybe 10 or 20 percent—and in others they are large, as in the DARPA study. Vince Clark, a University of New Mexico psychology professor who was involved with the DARPA work, told me that he’d tried every data-crunching tactic he could think of to explain away the effect of tDCS. “But it’s all there. It’s all real,” Clark said. “I keep trying to get rid of it, and it doesn’t go away.”

Now the intelligence-agency version of DARPA, known as IARPA, has created a program that will look at whether brain stimulation might be combined with exercise, nutrition, and games to even more dramatically enhance human performance. As Raja Parasuraman, a George Mason University psychology professor who is advising an IARPA team, puts it, “The end goal is to improve fluid intelligence—that is, to make people smarter.”

Whether or not IARPA finds a way to make spies smarter, the field of brain stimulation stands to shift our understanding of the neural structures and processes that underpin intelligence. Here, based on conversations with several neuroscientists on the cutting edge of the field, are four guesses about where all this might be headed.

1. Brain stimulation will expand our understanding of the brain-mind connection.

The neural mechanisms of brain stimulation are just beginning to be understood, through work by Michael A. Nitsche and Walter Paulus at the University of Göttingen and by Marom Bikson at the City College of New York. Their findings suggest that adding current to the brain increases the plasticity of neurons, making it easier for them to form new connections. We don’t imagine our brains being so mechanistic. To fix a heart with simple plumbing techniques or to reset a bone is one thing. But you’re not supposed to literally flip an electrical switch and get better at spotting Waldo or learning Swahili, are you? And if flipping a switch does work, how will that affect our ideas about intelligence and selfhood?

Even if juicing the brain doesn’t magically increase IQ scores, it may temporarily and substantially improve performance on certain constituent tasks of intelligence, like memory retrieval and cognitive control. This in itself will pose significant ethical challenges, some of which echo dilemmas already being raised by “neuroenhancement” drugs like Provigil. Workers doing cognitively demanding tasks—air-traffic controllers, physicists, live-radio hosts—could find themselves in the same position as cyclists, weight lifters, and baseball players. They’ll either be surpassed by those willing to augment their natural abilities, or they’ll have to augment themselves.

2. DIY brain stimulation will be popular—and risky.

As word of research findings has spread, do-it-yourselfers on Reddit and elsewhere have traded tips on building simple rigs and where to place electrodes for particular effects. Researchers like the Wright State neuroscientist Michael Weisend have in turn gone on DIY podcasts to warn them off. There’s so much we don’t know. Is neurostimulation safe over long periods of time? Will we become addicted to it? Some scientists, like Stanford’s Teresa Iuculano and Oxford’s Roi Cohen Kadosh, warn that cognitive enhancement through electrical stimulation may “occur at the expense of other cognitive functions.” For example, when Iuculano and Kadosh applied electrical stimulation to subjects who were learning a code that paired various numbers with symbols, the test group memorized the symbols faster than the control group did. But they were slower when it came time to actually use the symbols to do arithmetic. Maybe thinking will prove to be a zero-sum game: we cannot add to our mental powers without also subtracting from them.