Approximately one out of every 100 people has an issue with speech fluency. Although the most common speech disorder is persistent developmental stuttering, little is known about its causes. Scientists have now identified a hyperactive network in the right frontal part of the brain which plays a crucial role in this disorder by inhibiting speech, movement planning, and execution.
Speech disorders affect one percent of adults and five percent of children. Previous studies have attributed stuttering to imbalanced activity of the two brain hemispheres, in which a region in the left frontal brain is hypoactive and the corresponding region in the right hemisphere is hyperactive.
Experts at the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig and the University Medical Center Göttingen set out to investigate the cause of this imbalance. They found that the hyperactivity in the right hemisphere is the central cause of stuttering.
“Parts of the right inferior frontal gyrus are particularly active when we stop actions, such as hand or speech movements,” said study first author and neuroscientist Nicole Neef. “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.”
This hyperactivity impairs the left inferior frontal gyrus, which processes the planning of speech movements. The overactivity also hinders the left motor cortex, which regulates speech movements.
“If these two processes are sporadically inhibited, the affected person is unable to speak fluently,” explained Neef.
The scientists examined these processes using Magnetic Resonance Imaging (MRI) in adults who have experienced stuttering since childhood.
Participants were asked to imagine themselves saying the names of the months. This method of imaginary speaking was used to ensure that real speech movements did not interfere with the sensitive MRI signals. The neuroscientists then scanned for modified fibre tracts in the overactive right hemisphere regions.
The researchers discovered a fibre tract in the right network that was much stronger in the individuals with speech disorders.
“The stronger the frontal aslant tract, the more severe the stuttering. From previous studies we know that this fibre tract plays a crucial role in fine-tuning signals that inhibit movements,”said Neef. “The hyperactivity in this network and its stronger connections could suggest that one cause of stuttering lies in the neural inhibition of speech movements.”