By Anne L. Foundas, MD, Summer 2001
For the past four years, our research group has been interested in learning more about the biological basis of stuttering. In earlier studies, our group and others have found that specific brain regions that mediate speech, language and motor functions are asymmetric or larger in one hemisphere than the other, and the larger region is often functionally dominant.
So, the left side of the brain is usually dominant for language functions, and the plenum temporale, a portion of language cortex, is larger in the left hemisphere in most right-handed adults. Frontal cortical language areas, like the pars triangularis and pars opercularis, have also been found to be larger in the left hemisphere in most adults.
There is some evidence that these brain regions are atypical in individuals with developmental language disorders, like dyslexia and specific language impairment; but these regions have not been studied in individuals who stutter. Atypical anatomy can be defined as atypical asymmetry, or atypical size. In addition, atypical bumps or gyri have been found. Our goal was to determine whether adults with perisitent developmental stuttering (PDS) have atypical anatomical features within cortical areas that mediate speech and language functions.
We studied sixteen adults with persistent developmental stuttering and sixteen fluent adults matched for age, gender, handedness, and education. Volumetric MRI scans were acquired, and anatomical brain regions that mediate speech and language functions were measured and the groups were compared.
We found that the right and left planum temporale was significantly larger with reduced interhemispheric size or more symmetry in the adults who stutter. Gyrification patterns also differed between the groups with some anatomical configurations unique to the adults who stutter. These included variants in the anatomy of the diagonal sulcus, and extra-gyri along the superior bank of the sylvian fossa.
Overall, the adults with PDS had significantly more atypical anatomical features (average of 4 per individual) than controls (average one per individual). Some distinct features were seen in men versus women and right versus left handers. All of the right-handed women who stutter had extra-gyri, but none had atypical planar asymmetry.
In contrast, the majority of right-handed men who stutter (6 of 9) had extra-gyri and atypical planar asymmetries (5 of 9). Left handed men who stutter had atypical planar asymmetries, but rarely had extra-gyri. Furthermore, 5 of 9 right handed men with PDS had a double diagonal sulcus. This unique anomaly only occurred in one left-handed and one right-handed female with PDS, and was never seen in a control.
These results provide strong evidence that adults with PDS have anomalous anatomy in perisylvian brain regions. No one anatomical feature distinguished the groups, but multiple loci within a widely distributed neural network differed between groups.
These results provide the first evidence that anatomic anomalies may put an individual at risk for the development of stuttering, and suggest that distinct features may be more common in men versus women, and right versus left handers who stutter.
Our research efforts are currently expanding to include functional MRI and treatment studies in adults who stutter. These proposed studies will provide a biological framework to learn more about the anatomy and functional activation of cortical regions in individuals who do and do not stutter, should provide information to develop targeted behavioral and pharmacological interventions, and may lead to earlier detection of individuals at risk for developmental stuttering.
Acknowledgments: This study was supported by NIH grant DC00135, PHS CRR Grant RR05096 Tulane-LSU General Clinical Research Center, and the Department of Veterans Affairs South Central MIRECC. These results will be presented at the 53rd Annual Meeting of the American Academy of Neurology, Philadelphia, PA, May 8, 2001, and are in press, Neurology. A Dana Foundation Grant has been awarded for the treatment study.