Default mode network abnormalities in mesial temporal lobe epilepsy: a study combining fMRI and DTI

Abstract

Studies of in mesial temporal lobe epilepsy (mTLE) patients with hippocampal sclerosis (HS) have reported reductions in both functional and structural connectivity between hippocampal structures and adjacent brain regions. However, little is known about the connectivity among the default mode network (DMN) in mTLE. Here, we hypothesized that both functional and structural connectivity within the DMN were disturbed in mTLE. To test this hypothesis, functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) were applied to examine the DMN connectivity of 20 mTLE patients, and 20 gender- and age-matched healthy controls. Combining these two techniques, we explored the changes in functional (temporal correlation coefficient derived from fMRI) and structural (path length and connection density derived from DTI tractography) connectivity of the DMN. Compared to the controls, we found that both functional and structural connectivity were significantly decreased between the posterior cingulate cortex (PCC)/precuneus (PCUN) and bilateral mesial temporal lobes (mTLs) in patients. No significant between-group difference was found between the PCC/PCUN and medial prefrontal cortex (mPFC). In addition, functional connectivity was found to be correlated with structural connectivity in two pairwise regions, namely between the PCC/PCUN and bilateral mTLs, respectively. Our results suggest that the decreased functional connectivity within the DMN in mTLE may be a consequence of the decreased connection density underpinning the degeneration of structural connectivity.

Copyright © 2010 Wiley-Liss, Inc.

Figures

Figure 1

Group‐level analyses of the DMN. A: DMN in the controls (T > 2.49, P < 0.05, FDR corrected). B: DMN in the mTLE patients (T > 2.74, P < 0.05, FDR corrected). C: Contrast between the DMN in mTLE patients and controls (|T| > 1.69, P < 0.05, clustering extent threshold, 15 voxels).

Figure 2

Example of DTI fiber tractography on one control and one patient. Only three fiber bunches located between the PCC/PCUN and mPFC, bilateral mTLs, were detected in both subjects. The color‐coding of the obtained fibers is based on the standard RGB code applied to the vector at every segment of each fiber. Red indicates the medio‐lateral plane. Green indicates the dorso‐ventral orientation. Blue indicates the rostro‐caudal direction.

Figure 3

Functional connectivity within the DMN in patients and controls. Averaged temporal correlation coefficient r across all subjects in the control group (A) and in the mTLE group (B).

Figure 4

Comparison of functional and structural connectivity from PCC/PCUN to the mPFC, left mTLs, and right mTLs between mTLE patients and controls. The ANOVA on mean path length (A), mean connection density (B), mean temporal correlation coefficient (C), mean (D), and maximum (E) FA values on the tracts showed significant differences between the two groups at P = 0.051, P < 0.004, P < 0.001, P < 0.001, and P < 0.001, respectively. For each connection, differences at the significant level of P < 0.05 Bonferroni corrected and P < 0.01 Bonferroni corrected were marked with * and **, respectively.

Figure 5

The relation between functional connectivity (y axis) and structural connectivity (x axis) within DMN. The mTLE and controls groups were considered separately. A: Correlation coefficient r = 0.465 in patients and r = 0.536 in controls on the connected PCC/PCUN and the left mTLs. B: Correlation coefficient r = 0.547 in patients and r = 0.590 in controls on the connected PCC/PCUN and the right mTLs. In all these cases, correlations were significant at P < 0.05, uncorrected.