It began more than a decade ago, as engineers and hobbyists started getting increasingly enthralled with a jury-rigged electric technology that purportedly enhanced brain function. The movement is still growing, and the brain-zappers are no longer just young garage tinkerers—now they include older professionals who fork over hundreds of dollars for high-grade wearable systems. As scientists, clinicians and industry leaders study these fascinating but controversial mind-altering practices, one thing is clear: The idea of applying an electrical current to the scalp to boost learning or treat medical conditions in the comfort of one’s own home is gaining traction. But does it work?

This form of attempted brain hacking—known as transcranial direct current stimulation (tDCS)—is not as far-fetched as it might sound. The brain runs on electricity. Brain cells build up charges that impel chemical signals across synapses, the tiny gaps between neurons. When we learn something, the synapses involved become conditioned to fire more readily, and tDCS supposedly enhances that process. The tiny electrical currents tDCS uses—generally one to two milliamps—cannot actually trigger the chemical impulse that crosses a synapse, but some researchers believe tDCS strengthens synaptic connections to make learning more efficient. Small lab studies suggest it can improve vigilance and reaction times. “You get more bang for your buck” by combining tDCS with conventional training, contends Marom Bikson, a professor of biomedical engineering at the City College of New York.

The dawn of the noninvasive brain stimulation movement is widely attributed to a 2000 paper by German neurophysiologists Michael Nitsche and Walter Paulus. In a small study of healthy twentysomethings the scientists showed they could make neurons more or less excitable by sending weak electrical currents through the brain’s motor cortex. The effects lingered minutes after stimulation. Previously tDCS had only been studied in animals.

A subsequent study in 2003 showed such stimulation could improve a cognitive ability psychologists call motor sequence learning—the process of training the brain in the precisely sequenced steps required to interact with the world via means such as listening or executing a movement. Several labs at Harvard Medical School and the National Institutes of Health picked up on these findings and conducted research suggesting tDCS was promising for stroke rehabilitation and chronic pain.

It was then, in 2006, that Brett Wingeier began watching the field. Trained in medical devices, Wingeier was the principal biomedical engineer at NeuroPace, a Silicon Valley company that makes brain implants intended to help control epilepsy. He had also worked on brain stimulators for alleviating cluster headaches and the movement symptoms that afflict patients with Parkinson’s disease. “They can be incapacitated, but you turn the stimulator on and it all goes away. It’s amazing,” Wingeier says. Yet he notes those devices are “only relevant to patients with relatively severe disorders. They’re surgical implants.”

Wingeier dreamed of using neurotechnology to help more patients—and maybe even healthy people. He was excited to see new tDCS studies, but many were “done with yellow sponges and measuring tape and straps on your head,” he says. The time seemed ripe to consider bringing product design principles into what he calls “a legitimate consumer product people can pull out of the box and use.”

More plans began taking shape as scientists fleshed out possible mechanisms for how tDCS works. In a set of electrophysiology experiments published in 2009, Bikson and some of his colleagues demonstrated that a mild electric field can synchronize activity in a neural network. Such insights inspired Wingeier and other researchers working to target tDCS to specific brain areas.

Since the turn-of-the-century study by Nitsche and Paulus researchers have published nearly 3,000 scientific papers on tDCS—a third of them in the last two years. In the medical realm some studies suggest brain stimulation might help people with neuropathic pain as well as depression, schizophrenia and a range of other psychiatric illnesses. For now tDCS remains highly experimental in the U.S. although in Europe such devices are approved for treating pain and depression. More than 500 tDCS studies are listed in the database that tracks ongoing trials for drug therapies or medical devices.

In addition, a growing number of studies are testing tDCS in healthy people and suggest it can improve working memory, attention and decision-making as well as stimulate creativity. A 2012 study funded by the U.S. Department of Defense indicated tDCS may heighten vigilance, making participants twice as fast at spotting bombs, snipers and other threats in fuzzy images. Four other labs have since shown the technology could enhance learning—“one of the few tDCS effects that’s been replicated across institutions,” says Vincent Clark, a cognitive neuroscientist and first author on the 2012 paper. Clark directs the Psychological Clinical Neuroscience Center at the University of New Mexico.

The tsunami of academic literature on tDCS has spurred a corresponding wave of media articles claiming jolts to the brain can improve memory, boost math skills and “bring out the genius in you.” An article in the Journal of Medical Ethics calls tDCS “the Swiss Army knife of human neuroscience.” As word of tDCS’s potential reaches the general public, home users have been convening in cyberspace. In 2011 tDCS-ers created a Reddit forum and launched a blog the following year. Thousands of home users visit these Web sites to discuss scientific papers, post how-to videos and ask questions such as where to place electrodes and how long to stimulate. The Reddit forum gets between 5,000 and 10,000 unique visitors per month and has grown from 2,500 to around 9,000 subscribers over the last three years.

In 2013 the first consumer tDCS wearable—a headset for gamers—appeared on the market. Since then more than a dozen companies have started selling tDCS products. Some offer $40 kits containing wires, electrodes and headbands that the customer assembles at home. At the other end of the spectrum, Halo Neuroscience—the San Francisco-based company Wingeier co-founded in 2013—sells $700 headgear aimed at elite athletes. The device stimulates the motor cortex and promises “gains in strength, explosiveness, endurance and muscle memory” when paired with workouts. Caputron, which launched in 2014, also sells a range of medical-grade consumer brain stimulation devices. The company distributed more than 10,000 tDCS products in 2015 and more than 20,000 in 2016.

Most consumer tDCS devices are not marketed for clinical purposes but for leisure and cognitive enhancement. None of the consumer kits have officially undergone the rigorous testing process that a drug or medical device must undergo for approval by the U.S. Food and Drug Administration, and the purchaser has only the manufacturer’s claim that it works for its intended use. And intended uses can be ambiguous. For example, when companies assert their tDCS device “increases concentration” or “amps up brain function,” it is hard to determine if such instruments fall under the FDA’s purview, says Anna Wexler, a PhD student at Massachusetts Institute of Technology who studies the ethical, legal and social implications of neurotechnology. The agency defines a medical device as “an instrument that is…intended to affect the structure or any function of the body,” she says. “The way a manufacturer markets the product determines the regulatory pathway,” adds Wexler, who explored some of these issues in a 2015 Journal of Medical Ethics paper. Enhancement claims have aroused suspicion among scientists, prompting more controlled lab studies. In one experiment, published in March 2016, a team from the Netherlands reported that a commercial tDCS headset actually made participants less accurate in a well-established test of working memory.

Despite uncertainty over efficacy claims, interest in tDCS as a tool for optimizing mental function persists. The devices are small and seem to be relatively safe. Surveying the literature on tDCS human trials, Bikson and colleagues reported this fall that conventional use has caused no serious adverse effects across more than 33,200 sessions, as well as 1,000 subjects with repeated sessions.

A major problem with assessing how well the technology works is that the vast majority of research studies use brain stimulation for different purposes than do home tDCS users. Contrast home usage with the seminal 2000 paper authored by Nitsche and his colleagues that tested whether noninvasive brain stimulation could help people recover movement after a stroke. “We just wanted to know if we could change motor performance,” Nitsche says. “Our question was not, ‘Does the stimulation protocol induce maximal performance?’” Home tDCS users, on the other hand, are not particularly interested in making new scientific discoveries, Wexler says. “They’re interested in self-improvement.”

This quest for enhancement could potentially lead to untoward consequences. In July clinicians and scientists who study tDCS published an Annals of Neurology editorial warning of potential undesired effects. For example, stimulating one brain area might help someone learn new material—but hurt the ability to process learned material. Changes in brain activity might last longer than users expect or may extend beyond the regions beneath the electrodes. Also, the effects of tDCS vary tremendously across individuals, and with small changes in the process. When it comes to placing electrodes, “even just a few centimeters can be enough to change the effect,” Clark says. “How long you stimulate, how much current you put in—if it’s two or one milliamp or something in between—all of that matters.” Genetics, age and skull thickness also seem to influence response. The technical specifications for medical uses have yet to be clearly delineated, let alone the parameters for cognitive enhancement.

This month at a New York City conference that Bikson helped organize, tDCS researchers and vendors will meet to discuss the technology’s potential as well as concerns surrounding it and other forms of neuromodulation. In the spring the group hopes to publish a report to help consumers make informed decisions about purchasing and using these devices. “Right now consumers are a bit confused. They see a $20 device, and they see one that costs $700,” Bikson says. “They might not have enough information to appreciate why the $700 device is a higher value for them.”

And it looks like tDCS is only the beginning. In addition to applying direct current to the scalp, brain-hacking researchers and DIYers are experimenting with other methods including alternating current and random noise stimulation. These might one day complement more established noninvasive medical technologies, such as magnetic stimulation and ultrasound, to influence brain function and behavior. Transcranial direct current stimulation, Clark says, “is just the tip of the iceberg.”