The importance of insula in speech control is acknowledged but poorly understood, partly due to a variety of clinical symptoms resulting from insults to this structure. To clarify its structural organization within the speech network in healthy subjects, we used probabilistic diffusion tractography to examine insular connectivity with three cortical regions responsible for sound processing [Brodmann area (BA) 22], motor preparation (BA 44) and motor execution (laryngeal/orofacial primary motor cortex, BA 4). To assess insular reorganization in a speech disorder, we examined its structural connectivity in patients with spasmodic dysphonia (SD), a neurological condition that selectively affects speech production. We demonstrated structural segregation of insula into three non-overlapping regions, which receive distinct connections from BA 44 (anterior insula), BA 4 (mid-insula) and BA 22 (dorsal and posterior insula). There were no significant differences either in the number of streamlines connecting each insular subdivision to the cortical target or hemispheric lateralization of insular clusters and their projections between healthy subjects and SD patients. However, spatial distribution of the insular subdivisions connected to BA 4 and BA 44 was distinctly organized in healthy controls and SD patients, extending ventro-posteriorly in the former group and anterio-dorsally in the latter group. Our findings point to structural segregation of the insular sub-regions, which may be associated with the different aspects of sensorimotor and cognitive control of speech production. We suggest that distinct insular involvement may lead to different clinical manifestations when one or the other insular region and/or its connections undergo spatial reorganization.
Keywords: Insular organization; Probabilistic tractography; Speech production.