Brain Structure and Function

, Volume 221, Issue 6, pp 3337–3345 | Cite as

Anatomy and lateralization of the human corticobulbar tracts: an fMRI-guided tractography study

  • Frédérique J. Liégeois
  • James Butler
  • Angela T. Morgan
  • Jonathan D. Clayden
  • Chris A. Clark
Original Article


The left hemisphere lateralization bias for language functions, such as syntactic processing and semantic retrieval, is well known. Although several theories and clinical data indicate a link between speech motor execution and language, the functional and structural brain lateralization for these functions has never been examined concomitantly in the same individuals. Here, we used functional MRI during rapid silent syllable repetition (/lalala/, /papapa/ and /pataka/, known as oral diadochokinesis or DDK) to map the cortical representation of the articulators in 17 healthy adults. In these same participants, functional lateralization for language production was assessed using the well-established verb generation task. We then used DDK-related fMRI activation clusters to guide tractography of the corticobulbar tract from diffusion-weighted MRI. Functional MRI revealed a wide inter-individual variability of hemispheric asymmetry patterns (left and right dominant, as well as bilateral) for DDK in the motor cortex, despite predominantly left hemisphere dominance for language-related activity in Broca’s area. Tractography revealed no evidence for structural asymmetry (based on fractional anisotropy) within the corticobulbar tract. To our knowledge, this study is the first to reveal that motor brain activation for syllable repetition is unrelated to functional asymmetry for language production in adult humans. In addition, we found no evidence that the human corticobulbar tract is an asymmetric white matter pathway. We suggest that the predominance of dysarthria following left hemisphere infarct is probably a consequence of disrupted feedback or input from left hemisphere language and speech planning regions, rather than structural asymmetry of the corticobulbar tract itself.


Corticobulbar tract Speech Lateralization Functional MRI Tractography Diffusion-weighted MRI 



We thank our research radiographer, Tina Banks, for scanning and our participants for taking part.

Supplementary material

429_2015_1104_MOESM1_ESM.doc (17 kb)
Supplementary material 1 (DOC 17 kb)


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© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  1. 1.Cognitive Neuroscience and Neuropsychiatry SectionUniversity College London Institute of Child HealthLondonUK
  2. 2.Department of Audiology and Speech PathologyUniversity of MelbourneMelbourneAustralia
  3. 3.Murdoch Children’s Research InstituteMelbourneAustralia
  4. 4.Developmental Imaging and Biophysics SectionUniversity College London Institute of Child HealthLondonUK

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