Researchers map the cause of stuttering in the brain.
One per cent of adults and 5% of children are unable to achieve what most of us take for granted, speaking without stuttering. Instead, they struggle with words, often repeating the beginning of a word. A neuronal sign of persistent developmental stuttering is the magnified coactivation of right frontal brain regions during speech production. Whether and how stuttering severity relates to the connection strength of these hyperactive right frontal areas to other brain areas is an open question. Now, a study led by researchers from the Max Planck Institute identifies a fibre tract in the hyperactive right network that is much stronger in people with a stutter than in those without speech disorders. The team state that their data shows that the stronger the frontal aslant tract, the more severe the stuttering. The opensource study is published in the journal Brain.
Previous studies show an imbalance of activity in the two brain hemispheres in people who stutter compared to fluent speakers; a region in the left frontal brain is hypoactive, whereas the corresponding region in the right hemisphere is hyperactive. However, the cause of this imbalance is unclear. The current study analyses white matter brain structures in adults who stutter and those who do not to show that stuttering severity is linked to the strength of white matter connections of hyperactive right frontal brain regions.
The current study uses newly developed structural connectivity neuroimaging to calculate voxel-wise correlations between connection strength and stuttering severity within tract volumes that originated from functionally hyperactive right frontal regions in 31 adults who stutter and 34 matched control participants. Results show that with increasing speech motor deficits the connection strength increased in the right frontal aslant tract, the right anterior thalamic radiation, and in U-shaped projections underneath the right precentral sulcus. Data findings show that, in contrast, with decreasing speech motor deficits connection strength increased in the right uncinate fasciculus.
The group state that this brain structure–behaviour relationship incorporates affected right frontal spatially separated cortical regions into disparate networks, pinpointing areas of pathology and advancing circuit-based interpretations of the neuronal basis of the stuttering disorder. They go on to add that the second valuable achievement of this study constitutes the findings of reduced white matter integrity in the bilateral superior longitudinal fasciculus and the right frontal aslant tract of adults who stutter.
The team surmise that their combined functional MRI–diffusion tensor imaging study disentangles different networks involved in the neuronal underpinnings of the speech motor deficit in persistent developmental stuttering. For the future the researchers state that theirs is the first study to bring such a brain–behaviour relationship into awareness with more studies needed to test this hypothesis.