A new study has discovered that a hyperactive network in the right frontal part of the brain plays a crucial role in persistent developmental stuttering, which is the most frequent speech disorder.
According to scientists at Germany’s Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, and the University Medical Center Göttingen, this network inhibits speech movement planning and execution, interrupting the flow of speech.
According to the researchers, about 1 percent of adults and 5 percent of children stutter. Previous studies found imbalanced activity of the two brain hemispheres in people who stutter compared to fluent speakers: A region in the left frontal brain is hypoactive, while the corresponding region in the right hemisphere is hyperactive.
However, the cause of this imbalance is unclear. Does the less active left hemisphere reflect a dysfunction and causes the right side to compensate for this failure? Or is it the other way around and the hyperactive right hemisphere suppresses activity in the left hemisphere and is therefore the real cause of stuttering?
The new study has helped scientists gain crucial insights. They say the hyperactivity in regions of the right hemisphere seems to be central for stuttering.
“Parts of the right inferior frontal gyrus are particularly active when we stop actions, such as hand or speech movements,” said Dr. Nicole Neef, a neuroscientist at the Max Planck Institute and first author of the new study. “If this region is overactive, it hinders other brain areas that are involved in the initiation and termination of movements. In people who stutter, the brain regions that are responsible for speech movements are particularly affected.”
Two of these areas are the left inferior frontal gyrus, which processes the planning of speech movements, and the left motor cortex, which controls the actual speech movements.
“If these two processes are sporadically inhibited, the affected person is unable to speak fluently,” she explained.
For the study, the scientists used magnetic resonance imaging (MRI) in adults who have stuttered since childhood. The participants imagined themselves saying the names of the months.
Researchers explained that they used this method of imaginary speaking to ensure that real speech movements did not interfere with the sensitive MRI signals.
The neuroscientists were then able to analyze the brain by scanning for modified fiber tracts in the overactive right hemisphere regions in participants who stutter.
And that’s what they found: A fiber tract in the hyperactive right network that was much stronger in affected persons than in those without speech disorders.
“The stronger the frontal aslant tract, the more severe the stuttering,” she said. “From previous studies we know that this fiber tract plays a crucial role in fine-tuning signals that inhibit movements. The hyperactivity in this network and its stronger connections could suggest that one cause of stuttering lies in the neural inhibition of speech movements.”
The study was published in the journal Brain.
Photo: Typically, the right inferior frontal gyrus (IFG) stops the flow of speech, whereas the left one supports it. In people who stutter, these two areas are conversely activated: The right IFG is overactive and shows tightened connections with the frontal aslant tract, which is a sign of a strengthened movement inhibition. This interrupts the flow of speech and might inhibit activity in the left IFG. Credit: MPI CBS.