There’s an easy way to electrically stimulate the brain that has become popular with researchers, DIY communities, and startups. This technique, called transcranial electrical stimulation, simply uses electrodes that are stuck to the scalp, with no brain surgery required.
One company claims transcranial electrical stimulation can cure depression. Another says it will give Olympic athletes an edge. The only issue: No one understands how it works. And that has led some people to question whether it works at all. “It’s definitely a controversial field,” says Myles Mc Laughlin, an assistant professor of neuroscience at the Belgian University KU Leuven.
Now, however, Mc Laughlin is proposing a solution to the mystery—yet his theory is also stirring up more debate.
The controversy springs from conflicting studies done with this type of stimulation, which includes both transcranial alternating current stimulation (tACS) and transcranial direct current stimulation (tDCS), depending on whether the setup uses alternating or direct current. In both methods, the assumption has been that carefully positioned electrodes on the scalp create a focused electric field in a certain part of the brain, causing brain cells to “fire” into action.
In the past decade, hundreds of studies done with tACS and tDCS have shown that such stimulation can impact people’s moods, cognition, and movements. “A lot of people have reported that they put electrodes on the head, and they see some effect on behavior. I think those effects are real,” Mc Laughlin tells IEEE Spectrum.
On the other hand, researchers have shown that not much electric current actually reaches the brain. For example, one experiment on human cadavers found that 75 percent of the current was shunted away by skin and skull. That study, led by NYU neuroscience professor György Buzsáki, found that typical tACS and tDCS current levels generated a very weak electric field in the brain that couldn’t make neurons fire.
But Mc Laughlin says his results “could resolve that paradox.”
He and his colleagues had been trying to help patients with tremors, and they had seen studies suggesting that tACS might be effective in controlling the shakes. In their initial experiments, they tried positioning the electrodes at various locations on the patients’ scalps, trying to directly target different brain regions—but, surprisingly, they didn’t see different impacts on the tremors. Then they tried putting the electrodes on the patients’ arms, so they definitely weren’t sending current directly into the brain—and saw the same impact on the tremors.
“We started to think that the brain is involved [in tACS], but it’s not directly stimulating the brain,” Mc Laughlin says. Their theory: The stimulation activates the network of peripheral nerves just under the skin, which relays the signal to the brain.
That initial research led to the experiments just described in Nature Communications. Mc Laughlin’s team knew that tACS, applied with a certain frequency, has been shown to make a person’s natural tremors fall into sync with the stimulation. So they put accelerometers on their volunteers’ middle fingers to record the tremor rhythm, giving them a clear readout of whether the stimulation was having an effect.
In a key experiment, they placed electrodes on the scalp directly over the motor cortex, the brain region that controls movements. Then they cranked up the current, and found that the tACS did indeed cause synchronized tremors. But when they rubbed topical anesthetic on the scalp before turning up the current, the tACS did not cause synchronized tremors—suggesting that the numbed nerves under the skin weren’t conveying the signal to the brain.
Mc Laughlin is cautious in interpreting his results, and stresses that he only tested tACS’s effect on the motor system. There are still big open questions, he notes: For one, it’s not yet clear whether the same mechanism is at work in tDCS. He also wants to see similar experiments done on other brain systems, such as those involved in perception, cognition, and memory.
Buzsáki of NYU, who conducted the cadaver study, calls Mc Laughlin’s research “a step forward.” However, he says, the peripheral nerves may not be acting alone. “The demonstration of peripheral stimulation cannot exclude the possibility of a direct brain effect [as well],” he says. Even if tACS and tDCS directly generate only a weak electric field in the brain, he says, it’s essential to understand what impact that has.
Marom Bikson, a professor of neural engineering at the City College of New York and a leading tDCS researcher, also takes a tone of cautious curiosity. “Care must be taken in generalizing the conclusions to all tACS studies,” he says. “Regardless, this study justifiably increases scrutiny across transcranial electrical stimulation studies, including tDCS, to link empirical outcomes to the engagement of a central (versus peripheral) neurophysiological target.”
The controversy may not be resolved, but at least the research community has a new tool to employ going forward: a simple tube of topical anesthetic.