The new therapy aims to stimulate the brain with small currents applied to the scalp. Illustration by Harry Campbell

“What does this part of the brain do, again?” I asked, pointing to the electrode on my right temple.

“That’s the right inferior frontal cortex,” said Vince Clark, the director of the University of New Mexico Psychology Clinical Neuroscience Center, in Albuquerque. “It does a lot of things. It evaluates rules. People get thrown in jail when it’s impaired. It might help solve math problems. You can’t really isolate what it does. It has emotional components.”

It was early December, and night was falling, though it was barely five. The shadows were getting longer in the lab. My legs felt unusually calm. Something somewhere was buzzing. Outside the window, a tree stood black against the deepening sky.

“Verbal people tend to get really quiet,” Clark said softly. “That’s one effect we noticed. And it can do funny things with your perception of time.”

The device administering the current started to beep, and I saw that twenty minutes had passed. As the current returned to zero, I felt a slight burning under the electrodes—both the one on my right temple and another, on my left arm. Clark pressed some buttons, trying to get the beeping to stop. Finally, he popped out the battery, the nine-volt rectangular kind.

This was my first experience of transcranial direct-current stimulation, or tDCS—a portable, cheap, low-tech procedure that involves sending a low electric current (up to two milliamps) to the brain. Research into tDCS is in its early stages. A number of studies suggest that it may improve learning, vigilance, intelligence, and working memory, as well as relieve chronic pain and the symptoms of depression, fibromyalgia, Parkinson’s, and schizophrenia. However, the studies have been so small and heterogeneous that meta-analyses have failed to prove any conclusive effects, and long-term risks have not been established. The treatment has yet to receive F.D.A. approval, although a few hospitals, including Beth Israel, in New York, and Beth Israel Deaconess, in Boston, have used it to treat chronic pain and depression.

“What’s the plan now?” Clark asked, unhooking the electrodes. I could see he was ready to answer more questions. But, as warned, I felt almost completely unable to speak. It wasn’t like grasping for words; it was like no longer knowing what words were good for.

Clark offered to drive me back to my hotel. Everything was mesmerizing: a dumpster in the rear-view camera, the wide roads, the Route 66 signs, the Land of Enchantment license plates.

After some effort, I managed to ask about a paper I’d read regarding the use of tDCS to treat tinnitus. My father has tinnitus; the ringing in his ears is so loud it wakes him up at night. I had heard that some people with tinnitus were helped by earplugs, but my father wasn’t, so where in the head was tinnitus, and were there different kinds?

“There are different kinds,” Clark said. “Sometimes, there’s a real noise. It’s rare, but it happens with dogs.” He told me a story about a dog with this rare affliction. When a microphone was placed in its ear, everyone could hear a ringing tone—the result, it turned out, of an oversensitive tympanic membrane. “The poor dog,” he said.

We drove the rest of the way in silence.

Growing up in Detroit, Clark was interested in philosophy and thought he would study it in college. But, after realizing that all the questions that interested him came down to perception and the brain, he majored in psychobiology, at U.C.L.A. This was in the nineteen-eighties. “By luck, I picked a field that was about to explode,” he said.

As an undergraduate, Clark took a job at a hospital, building electrodes for insertion into the brains of epileptics during surgery, to locate the epileptic regions of the brain and the regions necessary for cognitive function. The patient’s head would be sawed open under local anesthetic. Fully conscious, the patient would be shown flashcards with words or pictures while the electrodes recorded which regions responded to the stimuli. Clark was deeply impressed by how localized neuronal responses were. Sometimes, a picture of a particular celebrity would cause a single neuron to become especially active. Similar observations led scientists in a later study to posit the existence in one patient of a “Halle Berry neuron.”

Just before Clark got his Ph.D., the fMRI machine was developed—a huge moment for neuroscience. The technology measures brain activity in real time, by monitoring blood flow. Scientists today can look at an fMRI and see what happens in the brain of a pianist playing Bartók, a Carmelite nun having a religious experience, a depressed person contemplating suicide, or a schizophrenic hearing voices. As a professor at the University of Connecticut Health Center, Clark began working on an addiction study, using fMRI to look at the brains of recovering addicts. To his surprise, he noticed that the fMRI could show which of the recovered addicts were likely to relapse in six months. Clark believes that it may be possible to stimulate a relapser’s brain with tDCS to make it look and act more like a non-relapser’s.

The precise physical mechanism of tDCS remains mysterious. The electric current used is too low to cause resting neurons to fire. Instead, it seems to make neurons more or less likely to fire, by changing the electrical potential of nerve-cell membranes. In other words, although tDCS can’t create new neural activity, it can enhance or reduce existing activity. The procedure uses direct current, so it has positive and negative electrodes and can have both inhibitory and excitatory effects: in general, positive current stimulates neural activity while negative current inhibits it.

Clark began working on tDCS in 2007, shortly after being named scientific director of the Mind Research Network at the University of New Mexico. Funded by DARPA, the research division of the Department of Defense, his first study determined that tDCS can help subjects learn to detect hidden threats in complex images. The researchers used images from DARWARS, a video game designed to familiarize Army recruits with the desert roads, derelict apartment blocks, and abandoned fruit markets that are apparently typical of the Middle Eastern landscape. For most people, the concealed threats—an explosive device hidden behind an oil drum; the shadow of a sniper’s rifle protruding over a rooftop—can be identified only with training and practice. At the beginning of the study, subjects’ brains were scanned by fMRI while they received training, to show which regions were active during learning. These areas were then targeted by electrodes in a new group of subjects as they performed the same task. Half of them received active tDCS; the other half, the control group, received “sham tDCS”—a negligibly low dose.

To Clark’s disbelief, the subjects who received tDCS learned the same material twice as quickly as the control group. The study was replicated by other labs, with similar results. The Air Force found that tDCS made airmen twice as accurate at identifying tanks and missile launchers in radar scans.

“It’s a huge, huge effect,” Eric Claus, a neuroscientist at the Mind Research Network, told me of the original results. “As cognitive neuroscientists, we rarely see effects that large.”

On hearing of Clark’s findings, Claus decided to incorporate tDCS into his own work: the treatment of alcoholism using cognitive exercises. He is currently replicating a study in which alcoholics were found to drink less after repeatedly using a joystick to push away images of beverages. Claus scans the brains of alcoholics while they perform the joystick task; he then uses tDCS to stimulate the active regions on a new group of alcoholics. Two members of the tDCS group have gone from drinking a fifth of liquor a day to not drinking at all.