Abstracts

Rationale: Increasing evidence has shown that structural damage in temporal lobe epilepsy (TLE) is not limited to the temporal lobes, but is often distributed across multiple brain networks. The functional organization of these networks can be observed in vivo by functional connectivity MRI (fcMRI) analysis. FcMRI evidence regarding the direction and the extent of changes in the hippocampal networks has been equivocal. There is also limited information available on the co-localization of structural and functional connectivity impairments in epilepsy despite the evidence that combining these two modalities can enrich our understanding of canonical brain networks. Methods: T1- and diffusion-weighted (DW) MRI data were collected on 15 healthy controls and 14 patients with refractory TLE. Diffusion tensor imaging (DTI) data were used to obtain information on fiber tract structure using a probabilistic DTI atlas. Functional connectivity data were derived from a task-regressed event-related semantic decision task. Bilateral hippocampal seed regions were used to generate whole-brain correlation maps, which were then subjected to group-level statistical analyses. Results: Compared to healthy controls, we observed a reduced correlation between bilateral hippocampi and the posterior cingulate region in patients with TLE (p < 0.0005; cluster size ≥ 5 voxels). The reduction extended towards the precuneus and was more extensive for the left-hippocampal network. Patients with TLE also exhibited an increased correlation between the anterior superior frontal gyrus and right hippocampus. A group difference trend in structural connectivity was also observed in the right hemisphere for the superior longitudinal fasciculus (SLF), inferior longitudinal fasciculus (ILF), and the cingulum fiber fractional anisotropy (FA; all p-values < .05). The former appeared to be driven by control vs. left TLE FA differences while the latter two were driven by control vs. right TLE FA differences. We also observed an association between the strength of the functional connectivity in the posterior cingulate and the FA of the cingulum. Conclusions: Our fcMRI data reveal bilateral reductions in connectivity between the hippocampus and the posterior cingulate region, known to be part of the default mode network (DMN). These reductions in functional connectivity co-localize with reductions in structural integrity of the SLF, ILF and the cingulum. While these results agree with previous findings of reduced DMN functional connectivity in patients with TLE, hippocampal specificity of the findings and the directionality of the relationship between co-localizing structural and functional connectivity changes remains uncertain.