Functional and structural changes in the memory network associated with left temporal lobe epilepsy

Abstract

Understanding functional plasticity in memory networks associated with temporal lobe epilepsy (TLE) is central to predicting memory decline following surgery. However, the extent of functional reorganization within memory networks remains unclear. In this preliminary study, we used novel analysis methods assessing network-level changes across the brain during memory task performance in patients with TLE to test the hypothesis that hippocampal functions may not readily shift between hemispheres, but instead may show altered intra-hemispheric organization with unilateral damage. In addition, we wished to relate functional differences to structural changes along specific fibre pathways associated with memory function. Nine pre-operative patients with intractable left TLE and 10 healthy controls underwent functional MRI during complex scene encoding. Diffusion tensor imaging was additionally performed in the same patients. In our study, we found no evidence of inter-hemispheric shifts in memory-related activity in TLE using standard general linear model analysis. However, tensor independent component analysis revealed significant reductions in functional connectivity between bilateral MTL, occipital and left orbitofrontal regions among others in left TLE. This altered orbitofrontal activity was directly related to measures of fornix tract coherence in patients (P < 0.05). Our results suggest that specific fibre pathways, potentially affected by MTL neurodegeneration, may play a central role in functional plasticity in TLE and highlight the importance of network-based analysis approaches. Relative to standard model-based methods, novel objective functional connectivity analyses may offer improved sensitivity to subtle changes in the distribution of memory functions relevant for surgical planning in TLE.

Network Construction and Graph Theoretical Analysis of Functional Language Networks in Pediatric Epilepsy

“…Controls tend to compartmentalize the brain into several smaller connected components as opposed to employing the whole brain to achieve a single language task. This decrease in efficiency agrees with other studies that demonstrated effects of epilepsy on memory networks (Campo et al, 2013;Vlooswijk et al, 2011;Voets et al, 2009), on resting state networks (Bettus et al, 2010;2009;Doucet et al, 2012;Liao et al, 2010a;Mankinen et al, 2012;Morgan et al, 2010;Waites et al, 2006;Z. Zhang et al, 2010), and on language networks Vlooswijk et al, 2010).…”

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

“…Widespread fractional anisotropy (FA) reduction was also found in the ipsilateral temporal lobe of the unilateral mTLE patients [Focke et al, 2008]. More recently, a combination of BOLD functional connectivity and DTI structural connectivity has been used to observe the memory network changes in mTLE patients [Voets et al, 2009]. Hence, the DMN disruption in mTLE, reflected in functional and structural connectivity properties, may be worthy of consideration.…”

Investigations of Brain Network Alterations in Epilepsy Using Functional Magnetic Resonance Imaging

“…In parallel to the functional connectivity work being pursued, a large effort has been devoted to examining the same issues in relation to MRI measured structural connectivity (Focke, et al 2008). The comparison between functional and structural connectivity in epilepsy may provide answers that neither method can individually (Voets, et al 2009). We believe that large scale neuroimaging studies that incorporate both structural and functional imaging with genetic, physiological and neuropsychological testing, like those for controls (The Human Connectome Project, www.humanconnectome.org) and Alzheimer's Disease (Alzheimer's Disease Neuroimaging Initiative (ADNI), University of California, San Francisco, USA) already underway, may provide the greatest potential in uncovering the mechanisms and effects of network alterations in the brain in epilepsy.…”