About 1% of the world’s adult population, around 70 million people, is thought to be affected by stuttering, which can impair quality of life and cause social distress and stigma. But its causes are still poorly understood.
When Guillermo Mejias was 7, his parents sent him to buy bread during a family vacation 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 time came, he wasn’t able to produce a single word. He remembers coming back empty-handed, feeling ashamed and wondering what to say to his parents. “I was so tense that I had inadvertently bit my cheeks and tongue and my mouth was bleeding“, He says.
Mejias is still suffering from stuttering, but today, as a brain researcher at the Complutense University of Madrid, he is studying ways to treat the problem. He is part of a group growing number of researchers who have pinned their hopes on non-invasive brain stimulation, a set of techniques that applies small electric currents to specific regions of the brain.
A few years ago, Mejias tried such a technique on himself in a one-time experiment that temporarily reduced the frequency of his stuttering, he says. Some randomized trials, including one published this month in the Journal of Fluency Disorders, also suggest that brain stimulation may benefit people who stutter. “I think brain stimulation is the future”Mejias says.
Not everyone agrees. No studies have yet reported effects lasting months after treatment, let alone years. And anecdotal evidence suggests that trials of stuttering treatments are susceptible to the placebo effect, meaning that, just by being in the trial, participants can see positive effects that aren’t due to the treatment.
Stuttering and treatments
The only treatment available, speech therapy, can improve fluency to some extent and help avoid the 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 like treating Parkinson’s disease with physiotherapy”.
Non-invasive brain stimulation, however, stimulates or directly inhibits neurons in specific regions of the brain in an attempt to remodel the circuitry 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 on stuttering by using various techniques to apply electrical stimulation to the circuits involved in speech. Later, participants could perform an activity that reactivates those same regions, 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 stimulation can help remodel problem circuits. Although electrical currents can cause slight discomfort, noninvasive brain stimulation is widely considered safe.
For his PhD, Mejias studied the effects of a technique called transcranial magnetic stimulation (TMS), in which a magnetic field is applied through a small coil placed on the participant’s head, inducing a small electrical current in the brain. The coil is only a few millimeters wide, allowing the stimulation to be targeted to specific areas of the brain. Mejias focused on the extra motor area, a region of the cerebral cortex involved in movement control that exhibits abnormal functioning in people who stutter.
Since the study was largely exploratory and to avoid the bureaucracy of a larger process, Mejias decided to test the TMS on himself. In a short article published in 2019, he reported that the treatment reduced blocks and repetitions in his speech by 30%. “I felt a progressive improvement, which also helped me build my confidence“, He says. “The effects were noticeable for a few weeks, but we didn’t track 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 stimulation occurs than in TMS, but the equipment required is much simpler and cheaper. “If it works, it’s something that speech and language pathologists can use very easily and safely,” says Watkins. Another benefit: it often causes less discomfort than TMS.
In a randomized, double-blind, 30-participant study published in 2018, Watkins and colleagues applied continuous currents for 20 minutes for five consecutive days. They aimed to stimulate the left lower frontal cortex, a brain region involved in speech and language processing that exhibits reduced activity in people who stutter. To control the placebo effect, half of the participants received dummy stimulation with a tiny current, enough to make them feel anything, but too weak to induce an effect.
The benefit was statistically significant, but modest: After 1 week, participants who received brain stimulation stammered 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 stuttering syllables dropping from 8.5% to 5.4% after stimulation.
“The results of the improvement are evident and similar in both”says Julio Prieto, head of the Clinical Neurophysiology Service at 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 limits”he adds: a few treatment sessions and a short follow-up period, making it impossible to tell if the effects last long enough.
Prieto says the field would benefit from more standardized protocols. Like many noninvasive brain stimulation studies, stuttering experiments vary in design and equipment. The tests also target different regions of the brain and use currents of different intensity and duration. “And then there are also factors that you cannot control, such as sex differences, participant readiness and even genetic variations”says Prieto. Several studies suggest that there is great variability in responses to brain stimulation among individuals.
Dennis Drayna, a U.S. National Institutes of Health geneticist emeritus who works on stuttering, says skepticism is in order. “Dozens of different treatments for stuttering have been tried, often with spectacular initial success”he claims. “None of these lasted.”
Brain stimulation researchers hope to persuade skeptics. Mejias is seeking funding to expand his personal experiment into a larger study. Watkins and his team are planning to try another type of electrical stimulation called AC transcranial stimulation, in which the amplitude of the current is not fixed but fluctuates. “It mimics the natural rhythms of the brain,” says Birtan Demirel, a researcher at Watkins’ lab who he stammers too.
Mejias is optimistic about treating stuttering at the root. “The brain was almost mythical, no one knew how it worked. Now we are starting to understand it and in the future we will be able to modify it “.
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