Synopsis

Task fMRI is routinely performed on patients for pre-surgical planning to localize critical components of the language network and assess hemispheric language dominance. However, the cortical representation of language in bilingual patients (those who are fluent in more than one language) is more complex than in monolinguals, and may involve overlapping but distinct sub-networks. The focus of our current study is to assess the functional connectivity strength of language networks derived from task fMRI activation maps obtained using paradigms performed in both primary native (L1) and secondary (L2) languages in a group of Arabic-English bilingual patients.

Purpose

Methods

From an overall study pool of 227 patients undergoing clinical presurgical fMRI mapping from year 2011 to year 2017, a total of five patients (age range 18-35 years, 3F, 2M, all right-handed) performed the SC task both in primary (Arabic) and secondary (English) languages. These patients had left hemispheric lesions, including one with left mesial temporal sclerosis, one with a left hemispheric cavernous malformation and 3 with low grade tumors. The age of English acquisition varied across subjects. Patient 1 had lack of overall familiarity with English. Patients 2 & 3 were early acquisition bilinguals (L2 acquired below the age of 7) while patients 4 & 5 were late acquisition bilinguals.

Imaging was performed on a 3.0 T Siemens Trio MRI with a 12-channel head matrix coil using 2D GE-EPI T2* weighted BOLD sequences for functional MR imaging (TR 2000ms/TE 30ms/64x64x33 matrix). The SC task is a covert block design paradigm involving alternating control and active blocks lasting 20 seconds each for a total task duration of 4 min. SPM12 was used for preprocessing of fMRI data (slice timing correction, realignment, normalization to MNI space at 2mm voxel resolution, and spatial smoothing using a 6 mm FWHM Gaussian kernel). T maps were obtained from the general linear model (GLM) analysis using the standard SPM canonical HRF (reflecting activation vs. rest).

Lateralization Index (LI) was computed as (L-R)/(L+R) where L refers to number of left and R number of right hemispheric suprathreshold activated voxels (T>3.0). LI ≥ 0.2 represents left hemispheric dominance, LI ≤ −0.2 represents right hemispheric dominance and (-0.2<LI<0.2) designates bilateral dominance³.

Graph-theory based network analysis was performed on task fMRI data to explore the connectivity strength and topology of activated language clusters in L1 & L2. Network nodes were defined by parcellating the language network into 34 regions of interest (ROIs) (17 for the left hemisphere and 17 for the right hemisphere) according to the Anatomical Automatic Labeling atlas (AAL)⁴. A representative mean time series for each region was extracted by averaging the time series of all voxels within that region of the thresholded task fMRI (T>3.0) activation map. The Pearson's correlation coefficients (r) were calculated between each possible pair of nodes’ time series to obtain the correlation matrix. The correlation matrix was further thresholded for graph density (r>0.4) and further analysis was performed on the weighted network of the undirected graph G (N, K) with N nodes and K edges. The following network measures of brain connectivity were investigated for both L1 and L2 language networks: average degree, small world properties⁵, characteristic path length, global clustering coefficient, global network efficiency⁶, and betweenness centrality.

Results

Table 1 (column 3) includes LI values from L1 and L2 task fMRI activation maps. Each patient demonstrated left hemispheric dominance (LI ≥ 0.2) with greater lateralization of L1 than L2 (see Figure 1). L2 displays higher average degree compared to L1 (Table 1, column 6).

In patients 2 & 3 (early acquisition bilinguals) the L2 language network displays higher global network efficiency as well as more pronounced small-worldness (i.e. short average path lengths and high clustering) structure compared to L1. In late acquisition bilinguals 4 & 5, L1 displays greater global efficiency and small-worldness compared to L2.

Patient No 1 had overall lack of familiarity with L2, which is reflected in very low betweenness centrality of L2 with respect to L1.

Discussion

Conclusion

Acknowledgements

References

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