Scientists are getting closer to revealing the full brain mechanism standing in the way of fluent speech.
Stuttering is a speech impairment wherein the fluency of speech is affected, with the speaker often repeating the syllables or sounds of one word compulsively. Stuttering tends to set in during early childhood, and over 5 percent of children experience it.
Stuttering usually goes away by adulthood, but it does persist in around 1 percent of the adult population and might lead to a decreased quality of life, depending on its severity.
By comparing the brains of adults who stutter and those who don't using MRI scans, scientists have observed that people whose speech is affected exhibit asymmetries in the brain regions responsible for verbal communication.
But so far, the exact brain mechanisms at play in have been mysterious. Now, researchers from the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig and the University Medical Center Göttingen — both in Germany — have uncovered fresh details about what happens in the brains of adults who stutter.
"Parts of the right inferior frontal gyrus are particularly active when we stop actions, such as hand or speech movements," explains first study author Nicole Neef, of the Max Planck Institute.
"If this region is overactive," she adds, "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."
The current study used MRI to identify the specifics of the mechanism related to hyperactivity in the right hemisphere of the brains of stuttering adults. Its findings were recently published in Brain: A Journal of Neurology.
'Fine-tuning signals' affected in stuttering
Neef and colleagues worked with 31 adults (15 females and 16 males), aged 36 years, on average, who had been stuttering since childhood, as well as 34 control participants (17 females and 17 males), aged 35.5 years, on average, who do not stutter.
All were free of neurological impairments and they did not use any drugs, and those who stuttered were matched as closely as possible for age, sex, education, and "handedness" — that is, whether they are right- or left-handed — with the controls.
The researchers thought that this speech impairment may be tied to the disruption of signals in the left inferior frontal gyrus, a region of the brain tied to the programming of physical speech movements, and in the left motor cortex, responsible for the regulation of these movements.
"If these two processes are sporadically inhibited, the affected person is unable to speak fluently," Neef observes.
To test their hypothesis, the researchers performed MRI scans on all the participants, comparing the results for the adults who stuttered with those obtained from the control group.
They asked the subjects to picture themselves uttering the names of the months, so that the brain activity related to speech could be examined without disrupting the MRI assessment.
Neef and colleagues specifically looked for altered fiber tracts — clusters of axons, the long fibers of brain cells that allow neurons to connect and communicate — in the brain regions related to speech in the right hemisphere of the brain.
They found that in the brains of those who stuttered, the frontal aslant tract — a neural pathway that has been tied to speech fluency — sent more signals than it normally would in the brain of a person without stutter, thus causing more activity at the destination.
"The stronger the frontal aslant tract," explains Neef, "the more severe the stuttering. 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," she continues, "could suggest that one cause of stuttering lies in the neural inhibition of speech movements."
The findings offer brand new insights into the brain mechanisms that are responsible for stuttering, focusing particularly on the role of an overactive right hemisphere in preventing the natural flow of speech-related physical movements.