When Guillermo Mejias was 7 years old, his parents sent him out to buy bread during a family holiday in southern Spain. Mejias still remembers his growing anxiety as he walked to the bakery, repeating what he would say over and over in his head. But when the moment arrived, he was unable to produce a single word. He recalls returning empty-handed, ashamed, and wondering what to tell his parents. “I was so tense that I had been inadvertently biting my cheeks and tongue and my mouth was bleeding,” he says.
Mejias still stutters, but today, as a brain researcher at the Complutense University of Madrid, he investigates ways to treat the problem. He is part of a growing group of researchers who have pinned their hopes on noninvasive brain stimulation, a set of techniques that applies small electric currents to specific brain regions. A few years ago, Mejias tried such a technique on himself in a one-off experiment that temporarily reduced the frequency of his stuttering, he says. A few randomized trials, including one published this month in the Journal of Fluency Disorders, also suggest brain stimulation can benefit people who stutter. “I think brain stimulation is the future,” Mejias says.
Not everyone agrees. No study has yet reported effects lasting months after treatment, let alone years. And anecdotal evidence suggests trials of stuttering treatments are susceptible to the placebo effect, meaning that, just from being in the trial, participants may see positive effects that aren’t due to the treatment.
About 1% of the world’s adult population—some 70 million people—is thought to stutter, which can impair quality of life and cause social distress and stigma. But its causes are still poorly understood.
The only available treatment, speech therapy, can improve fluency to some extent and help avoid side effects of stuttering, such as involuntary movements and tics. But its success rate is limited in adults and it fails to address the root of the problem, Mejias says: “Treating stuttering with speech therapy is akin to treating Parkinson’s disease with physiotherapy.”
Noninvasive brain stimulation, however, directly stimulates or inhibits neurons in specific brain regions in an effort to remodel circuits of interconnected cells. It has already shown benefits for depression—particularly in people who are resistant to drug treatments—and in stroke rehabilitation, where it can help improve movement and speech.
Mejias and others test its effects in stuttering by using various techniques to apply electrical stimulation to circuits involved in speech. Next, participants might perform a task that activates those same regions again, such as reading a text in unison with another person or with a metronome, which many people who stutter can do with some degree of fluency.
The hypothesis is that performing these tasks after the stimulation can help reshape the troublesome circuits. Although the electrical currents can cause slight discomfort, noninvasive brain stimulation is widely considered safe.
For his Ph.D., Mejias investigated the effects of a technique called transcranial magnetic stimulation (TMS), in which a magnetic field is applied through a small coil placed on the head of the participant, inducing a small electric current in the brain. The coil is only a few millimeters wide, allowing the stimulation to be targeted to specific brain areas. Mejias focused on the supplementary motor area, a region of the cerebral cortex involved in movement control that shows abnormal functioning in people who stutter.
Because the study was largely exploratory—and to avoid the red tape of a larger trial—Mejias decided to test TMS on himself. In a short paper published in 2019, he reported that the treatment reduced the blocks and repetitions in his speech by 30%. “I felt a progressive improvement, which also helped me build up my confidence,” he says. “The effects were evident for a few weeks but we didn’t monitor them consistently so I don’t know exactly how long they lasted.”
Kate Watkins, a neuropsychologist at the University of Oxford, was the first to treat people who stutter with another technique called transcranial direct current stimulation (tDCS), which passes an electrical current from one side of the brain to the other. Because the electrodes used in tDCS are relatively large, researchers have less control over where the stimulation happens than with TMS, but the required equipment is much simpler and cheaper. “If it works, it’s something that speech and language pathologists can use very easily and safely,” Watkins says. Another benefit: It often causes less discomfort than TMS.
In a randomized, double-blind trial with 30 participants published in 2018, Watkins and colleagues applied direct currents during 20 minutes for five consecutive days. They aimed to stimulate the left inferior frontal cortex, a brain region involved in language processing and speech that shows reduced activity in people who stutter. To control for the placebo effect, half of the participants received a sham stimulation with a minuscule current—enough for them to feel something, but too weak to induce an effect.
The benefit was statistically significant, but modest: After 1 week, participants who received the brain stimulation stuttered 8% of their syllables, down from 11% at baseline, with no detectable changes in the control group. The study published this month—by researchers in Iran, Germany, and Canada—involved 50 participants and was similar in both design and outcome, with stuttered syllables going down from 8.5% to 5.4% after stimulation.
“The results of improvement are evident and similar in both,” says Julio Prieto, chief of the Clinical Neurophysiology Service of the Gregorio Marañón University General Hospital in Madrid, who also studies stuttering and has collaborated with Mejias. “From my point of view they both have the same limitations,” he adds: few treatment sessions and a short follow-up period, making it impossible to say whether effects last.
Prieto says the field would benefit from more standardized protocols. Like many noninvasive brain stimulation studies, stuttering experiments vary in design and equipment. Trials also target different brain regions and use currents of different intensity and duration. “And then there are also factors that you can’t control, such as sex differences, the particpant’s alertness, and even genetic variations,” Prieto says. Several studies suggest there’s a large variability in responses to brain stimulation between individuals.
Dennis Drayna, an emeritus geneticist at the US National Institutes of Health who works on stuttering, says skepticism is in order. “Dozens of different treatments for stuttering have been tried, often with spectacular initial success,” he says. “None of these have lasted.”
Brain stimulation researchers hope to persuade the skeptics. Mejias is seeking funding to expand his self-experiment into a larger study. Watkins and her team are planning to try yet another type of electrical stimulation called transcranial alternating current stimulation, in which the current’s amplitude is not fixed but oscillates. “It mimics the natural brain rhythms,” says Birtan Demirel, a researcher at Watkins’s laboratory who also stutters.
Mejias is optimism about treating stuttering at its roots. “The brain used to be something almost mythical, nobody knew how it worked. Now, we’re beginning to understand it and in the future we’ll be able to modify it.”