Modulation of medial temporal lobe activity in epilepsy patients with hippocampal sclerosis during verbal working memory

Abstract

It has been traditionally assumed that medial temporal lobe (MTL) is not required for working memory (WM). However, animal lesion and electrophysiological studies and human neuropsychological and neuroimaging studies have provided increasing evidences of a critical involvement of MTL in WM. Based on previous findings, the central aim of this study was to investigate the contribution of the MTL to verbal WM encoding. Here, we used magnetoencephalography (MEG) to compare the patterns of MTL activation of 9 epilepsy patients suffering from left hippocampal sclerosis with those of 10 healthy matched controls while they performed a verbal WM task. MEG recordings allow detailed tracking of the time course of MTL activation. We observed impaired WM performance associated with changes in the dynamics of MTL activity in epilepsy patients. Specifically, whereas patients showed decreased activity in damaged MTL, activity in the contralateral MTL was enhanced, an effect that became significant in the 600- to 700-ms interval after stimulus presentation. These findings strongly support the crucial contribution of MTL to verbal WM encoding and provide compelling evidence for the proposal that MTL contributes to both episodic memory and WM. Whether this pattern is signaling reorganization or a normal use of a damaged structure is discussed.

Mapping the neuropsychological profile of temporal lobe epilepsy using cognitive network topology and graph theory

“…Furthermore, specific cognitive abilities have been examined not only with paper-and-pencil tests, but with diverse imaging techniques. For example, impaired working memory performance in epilepsy has been reported in conventional neuropsychological studies [34] as well as in effective connectivity [35] and functional imaging studies [36, 37]. …”

Different structural correlates for verbal memory impairment in temporal lobe epilepsy with and without mesial temporal lobe sclerosis

“…The thickness measures from the left and right hemisphere were combined, that is, added to provide a single measure from each region. This approach was chosen (1) to account for the findings of functional imaging studies in healthy controls [Desgranges et al, 1998], which did not show a clear preference for the left or right side in verbal tasks but rather provided evidence that both sides are involved although differently; (2) to account for potential compensatory inter-hemispheric shifts of the verbal memory function, which has been described in TLE [Bettus et al, 2009; Campo et al, 2009; Wagner et al, 2008; Richardson et al, 2003]. …”

Remote Effects of Hippocampal Sclerosis on Effective Connectivity during Working Memory Encoding: A Case of Connectional Diaschisis?

“…Accordingly, the goal of the current study was to investigate the conjoint function of MTL and other functionally related brain regions involved in verbal WM as a large-scale network (Bressler and Menon 2010). We obtained whole-head magnetoencephalographic (MEG) recordings during a verbal WM task, which was designed to ensure that participants encoded words semantically (Campo et al 2005, 2009), as prior neuroimaging investigations have demonstrated that depth processing modulates MTL activity (Kapur et al 1994; Lepage et al 2000). …”