Brain structure and function in developmental stuttering and bilingualism

© 2011 Peter Howell, John Van Borsel and the authors of individual chapters. All rights reserved. Brain imaging studies of developmental stuttering are starting to reach some consensus regarding the underlying neural mechanisms. Here, we provide some background to the different methods used to obtain and analyse brain imaging data. We review the findings from such studies in people who stutter that show, relative to fluent-speaking controls, overactivity in the right hemisphere centered on the right inferior frontal gyrus (homologue of Broca’s area), underactivity of auditory areas in the temporal lobe and abnormal activation of subcortical structures involved in the control of movement, namely the cerebellum and the basal ganglia. Structural abnormalities in the frontal and temporal lobes and the white matter connections between them are also commonly reported in people who stutter and may be related to the functional abnormalities. Although there are many behavioural studies on stuttering in bilinguals, to our knowledge there are no reports of neuroimaging in such individuals. Findings in each field separately, however, might inform the other. For example, studies in bilinguals have debated the relative involvement of the left and right hemispheres in processing first and second languages and incomplete cerebral dominance for speech and language is a candidate theory to explain stuttering. In addition, recent work in bilingualism has implicated the basal ganglia in regulating language output control. This idea is in agreement with proposals that stuttering is caused by dysfunction in the basal ganglia, which leads to timing problems with speech production. Brain imaging studies of developmental stuttering and bilingualism will inform our understanding of the neural bases of speech, vocal learning, auditory-motor integration and the development of language representation in the bilingual brain. Moreover, the questions of whether and in what way learning a second language might interact with or affect stuttering should shed light on optimal strategies for the treatment of stuttering both in monolingual and in bilingual children.