Abstract
Objective
Temporal Lobe Epilepsy (TLE) affects brain networks and is associated with impairment of episodic memory. Temporal and extra-temporal reorganisation of memory functions are described in functional magnetic resonance imaging (fMRI) studies. Functional reorganisations have been shown at the local activation level, but network-level alterations have been under-investigated. We aim to investigate the functional anatomy of memory networks using memory fMRI and determine how this relates to memory function in TLE.
Methods
Ninety patients with unilateral TLE (43 left) and 29 controls performed a memory-encoding fMRI paradigm of faces and words with subsequent out-of-scanner recognition test. Subsequent memory event-related contrasts of words and faces remembered were generated. Psychophysiological interaction (PPI) analysis investigated tasks-associated changes in functional connectivity seeding from the mesial temporal lobes (MTLs). Correlations between changes in functional connectivity and clinical memory scores, epilepsy duration, age at epilepsy onset, and seizure frequency were investigated, and between connectivity supportive of better memory and disease burden. Connectivity differences between controls and TLE, and between TLE with and without hippocampal sclerosis were explored using these confounds as regressors of no interest.
Results
Compared to controls, TLE patients showed widespread decreased connectivity between bilateral MTLs and frontal lobes, and increased local connectivity between the anterior MTLs bilaterally. Increased intrinsic connectivity within the bilateral MTLs correlated with better out-of-scanner memory performance in both left and right TLE. Longer epilepsy duration and higher seizure frequency were associated with decreased connectivity between bilateral MTLs and left/right OFC and insula; connections supportive of memory functions. TLE due to HS was associated with greater connectivity disruption within the MTL and extra-temporally.
Significance
Connectivity analyses showed that TLE is associated with temporal and extra-temporal memory network reorganisation. Increased bilateral functional connectivity within the MTL and connectivity to OFC and insula is efficient, and is disrupted by greater disease burden.
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