Anatomy and lateralization of the human corticobulbar tracts: an fMRI-guided tractography study
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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.
KeywordsCorticobulbar tract Speech Lateralization Functional MRI Tractography Diffusion-weighted MRI
We thank our research radiographer, Tina Banks, for scanning and our participants for taking part.
- Brett MA, Anton J-L, Valabregue R, Poline JB (2002) Region of interest analysis using an SPM toolbox. In: Paper presented at the 8th international conference on functional mapping of the human brain, Sendai, JapanGoogle Scholar
- Filler A (2009) Magnetic resonance neurography and diffusion tensor imaging: origins, history, and clinical impact of the first 50,000 cases with an assessment of efficacy and utility in a prospective 5000- patient study group. Neurosurgery 65:A29–43. doi: 10.1227/01.neu.0000351279.78110.00 CrossRefPubMedPubMedCentralGoogle Scholar
- Kent RD, Kent JF (2000) Task-based profiles of the dysarthrias. Folia Phoniatr Logop 52:48–53. doi:21512Google Scholar
- Simonyan K, Ostuni J, Ludlow CL, Horwitz B (2009) Functional but not structural networks of the human laryngeal motor cortex show left hemispheric lateralization during syllable but not breathing production The Journal of neuroscience : the official journal of the Society for. Neuroscience 29:14912–14923. doi: 10.1523/JNEUROSCI.4897-09.2009 PubMedPubMedCentralGoogle Scholar
- Talairach JT, Tournoux P (1988) Co-planar stereotaxic atlas of the human brain. Thieme Medical, New YorkGoogle Scholar
- Vernooij MW, Smits M, Wielopolski PA, Houston GC, Krestin GP, van der Lugt A (2007) Fiber density asymmetry of the arcuate fasciculus in relation to functional hemispheric language lateralization in both right- and left-handed healthy subjects: a combined fMRI and DTI study. Neuroimage 35:1064–1076. doi: 10.1016/j.neuroimage.2006.12.041 CrossRefPubMedGoogle Scholar
- Wechsler D (1999) Wechsler Abbreviated Scale of Intelligence™ (WASI™) Pearson Assessment, London, UKGoogle Scholar