Several sprinters and hurdlers who will be competing for Olympic medals in Rio de Janeiro next month have been using a new training tool in preparation: stylish headphones that double as a brain stimulator.

The device looks and works like real headphones but is engineered to precisely deliver mild electric current to the motor cortex, an area of the brain that coördinates movement.

Dan Chao, cofounder and CEO of Halo Neuroscience, which developed the device and has been selling it for several months, says the company’s data shows that if elite athletes wear the stimulator while they train, it can make them better at specific athletic tasks—like accelerating off the starting blocks in a sprint or hurdle race.

Hafsatu Kamara, a 100-meter sprinter from Sierra Leone who will be competing in her first Olympics this summer, performs a training exercise while wearing Halo’s brain-stimulating headphones.

The company has been working with five athletes prepping for Rio: Hafsatu Kamara, a sprinter from Sierra Leone; Michael Tinsley, a hurdler from the U.S. who won a silver medal in 2012; Mike Rodgers, a relay sprinter from the U.S.; Mikel Thomas, a hurdler from Trinidad and Tobago; and Natasha Hastings, a relay sprinter from the U.S. who won Olympic gold in in 2008.

The stimulation technique, called transcranial direct current stimulation, is a very popular subject of neuroscience research at the moment. Scientists have shown that delivering a small amount of electrical current can make neurons more or less likely to fire. And in the past 15 years a flurry of studies have suggested that the approach can be used for many things, from improving cognition to helping stroke patients regain movement.

Most of the studies have been small, though, and for many potential applications there is not yet enough data to tell if the stimulation actually works. Halo chose to target the motor cortex because it’s the area where there’s the most evidence that the technique does improve learning, says Chao. Since it is not intended for a medical purpose, the device, called the Halo Sport, is not regulated by the U.S. Food and Drug Administration. And athletes who use it are not violating any existing Olympic rules.

But some researchers argue that marketing the technology directly to consumers is not appropriate because we don’t yet know enough about what it does to the brain. Though ample evidence suggests that transcranial direct current stimulation is safe as long as established protocols are followed, there could be downsides that aren’t clear yet, says Charlotte Stagg, head of the physiological neuroimaging group in the Department of Clinical Neurosciences at the University of Oxford.

As for the claim that the technology can enhance athletic performance: “I think it’s unlikely that we understand enough to be able to successfully use it for that kind of thing at the moment,” says Stagg. That’s because scientists understand its effects on the motor cortex only in the context of lab studies involving relatively simple tasks. Athletic training is more complex, involving more muscles and many brain regions, she says.

Chao says the results from Halo’s work with elite athletes suggests otherwise. “For us, our results are proof enough,” he says, adding that the company does plan to submit some of this data for scientific peer review in the near future.