A new study from Canada finds that compared with fluent children, those who stutter have less grey matter in regions of the brain that are important for producing speech.
The findings will greatly help understanding of the structure of the brain in relation to speech production and what happens to make people stutter.
They may also show the importance of getting treatment as early as possible.
The researchers, from the University of Alberta (U of A), report their work in a recent issue of the journal Cortex.
Previous studies have highlighted differences in brain structure between fluent and stuttering people, but these have mainly looked at scans of adult brains.
The problem with those findings is, because they are looking at brains many years after onset - typically stuttering starts between the ages of 2 and 5 years - they cannot tell you whether the brain differences are the cause of the stuttttering or result from it.
The study's lead researcher is speech-language pathologist Deryk Beal, who is assistant professor in the Department of Speech Pathology and Audiology at U of A. He explains:
"You can never be quite sure whether the differences in brain structure or function you're looking at were the result of a lifetime of coping with a speech disorder or whether those brain differences were there from the beginning."
For their study, the researchers took brain scans of 28 children aged from 5 to 12 years. Half the children were diagnosed with stuttering, the other half were fluent and served as controls.
They used "voxel-based morphometry" to measure differences in the amount of grey matter in the brains of the two groups of children.
They found the children who stuttered had less grey matter volume in some regions (namely the "bilateral inferior frontal gyri and left putamen") and more in others ("right Rolandic operculum and superior temporal gyrus"), compared with the fluent children.
The children who stuttered also had less white matter volume "bilaterally in the forceps minor of the corpus callosum," they noted.
Understanding motor-sequence learning
The region where the stuttering children had less grey matter, the inferior frontal gyrus, is thought to control the process of taking articulatory information about language and sounds and coding it into speech.
Prof. Beal, who is also executive director of the Institute for Stuttering Treatment and Research (ISTAR) in the Faculty of Rehabilitation Medicine at U of A, explains:
"If you think about the characteristics of stuttering - repetitions of the first sounds or syllables in a word, prolongation of sounds in a word - it's easy to hypothesize that it's a speech-motor-control problem."
"The type of stuttering treatment we deliver at ISTAR is delivered with this limitation of the speech system in mind, and we have good success in stuttering treatment," he adds.
Prof. Beal sees these findings as the first step toward a test that looks at how grey matter is influenced by stuttering treatment, one that can help us better understand the nature of motor-sequence learning differences between children who stutter and those who do not:
"The more we know about motor learning in these kids, the more we can adjust our treatment - deliver it in a shorter period of time, deliver it more effectively."
Funds from the Canadian Institutes of Health Research Clinical Fellowship and the Hospital for Sick Children's Clinician Scientist Training Program helped finance the research.
Most preschool children who stutter eventually grow out of it, but a recently published study suggests a wait-and-see approach can harm stuttering children who ultimately do not recover and set them back academically and socially.