Microstructural differences in the morphology of the corpus callosum between normal and interpreter bilinguals revealed by DTI

Microstructural differences in the morphology of the corpus callosum between normal and interpreter bilinguals revealed by DTI
S.G. Mohades1, E. Struys1 , R.Luypaert1,2 (VrijeUniversiteitBrussel1,UZ Brussel Radiology/BEFY2)
Introduction
Previous studies have shown a relationship between corpus callosum (CC) morphology and the lateralization of language processing and the correlation between the size of the CC and developmental language disorders. Various parts of the CC are associated with language. We used DT-MRI based tractography to investigate the morphology of the CC. We compare five bundles arising for the sub-regions of the mid-sagittal CC in simultaneous bilinguals and bilingual interpreters; 1) arising from anterior third of the CC connecting to the orbital lobe (OL), 2) form the anterior mid-body of the CC to the primary motor cortex (PMC), 3) from the mid-posterior part of the CC,4 ) from the posterior part of the CC and 5) connection to the temporal lobe.
Aim
The hypothesis of our study was that the sub-regions of the CC associated with language exhibit structural differences between bilinguals and interpreters and that MRI DTI can be used to assess these differences. The idea was that among multiple language users the microstructure of the brain of trained professional language switchers might be different from the normal bilinguals.
Materials and Methods
A 3T MR system (Philips Achieva Release 2.5) with an 8 channel SENSE head coil was used. 12 bilinguals and 12 interpreters (right-handed, healthy males and females, 21-27 year old), were scanned.
Scan parameters: SS-SEEPI, 15 non-collinear diffusion gradient directions,b= 700 s/mm2,TR/TE=6484ms/60ms, FOV=224x224x120 mm3, 60 oblique axial 2mm slices, scan duration=454s. Figure 1 shows 5 bundles: orbital, motor cortex, mid-posterior, posterior and temporal lobes. The mean fractional anisotropy (FA) in the bundles was calculated for each subject using DTIstudio (Johns Hopkins University, Baltimore, USA). An independent-samples t-test was undertaken to compare the mean FA values between groups. A P-value of less than 0.05 was considered significant.
Results and Discussion
The FA values are summarized in Table1. The results show that the mean FA value is significantly lower in interpreters compared to bilinguals in four of the bundles (PMC, mid-posterior, posterior and temporal). No significant difference between the two groups could be found for the FA values of fibres going from the CC to the orbital lobe.
These results support the hypothesis that the structure of the CC can be changed by training to switch between two languages.

Figure 1 Left: Regional subdivision of the mid-sagittal CC; Right: Fibers to the orbital lobe (green), primary motor cortex (yellow), temporal Lobe (pink) and from mid-posterior CC (purple) and posterior CC (orange).

Orbital Primary motor cortex Mid-posterior Posterior Temporal
Simultaneous bilinguals 0.602±0.041 0.607±0.021 0.612±0.021 0.684±0.014 0.685±0.028
interpreters 0.588±0.022 0.586±0.014 0.588±0.018 0.660±0.015 0.651±0.019
P-value of t-test 0.35 0.014* 0.014* 0.0128* 0.005*



Reference:Coggins PE.2004, Brain Lang 89(1):69-75,Mori S.2002, NMR in Biomedicine 15(7-8):468.Huang H. 2005 Neuroimage 26(1):195-205