Structural and functional brain abnormalities associated with developmental stuttering
Developmental stuttering (or stammering) is a disorder affecting the fluency of speech, characterized by pauses or blocks, and repetitions or prolongations of sounds. It affects approximately 1% of the adult population and 4% of children; at least four fifths of those affected are male. The study of persons who stutter (PWS) presents the scientist with a unique group of subjects who have a restricted neural system deficit and who are otherwise cognitively, psychiatrically and neurologically unimpaired. Furthermore, short-term fluency can be rapidly induced in PWS, allowing within-subject comparisons of stuttered and fluent speech. Stuttering appears to be primarily a motor-speech disorder, sharing many characteristics with other motor control disorders such as Tourette’s syndrome, dyspraxia, dysarthria and dystonia. We have reason to suspect abnormal function in cortical-striatal-thalamic loops in PWS: stuttering symptoms improve following the administration of dopamine antagonists e.g. haloperidol, risperidone and olanzapine and worsen after treatment with a dopamine agonist e.g. levodopa; acquired stuttering typically is associated with lesions in the basal ganglia or thalamus. Perceptual mechanisms, however, also appear to be involved in stuttering; symptoms are dramatically reduced when speaking during auditory masking conditions, in unison or under altered auditory feedback. Previous studies describe both structural and functional abnormalities in PWS but have not explored the relationship between these two sets of data. I will present brain function and structure data obtained in a group of adolescents and young adults who stutter and controls. Functional imaging data was acquired during reading of sentences under normal and altered feedback conditions. To examine white matter integrity, fractional anisotropy (FA) maps were constructed from diffusion-weighted images and analysed using a new method known as Tract-Based Spatial Statistics. Differences in FA were examined between PWS and control groups. Our preliminary results support the notion of an abnormality in the basal ganglia and dopamine in PWS and also suggest that some of the abnormal functional activation obtained during speech production in PWS is related to the integrity of the underlying white matter tracts.