Aberrant Functional Connectivity in Dissociable Hippocampal Networks Is Associated with Deficits in Memory

Abstract

In the healthy human brain, evidence for dissociable memory networks along the anterior-posterior axis of the hippocampus suggests that this structure may not function as a unitary entity. Failure to consider these functional divisions may explain diverging results among studies of memory adaptation in disease. Using task-based and resting functional MRI, we show that chronic seizures disrupting the anterior medial temporal lobe (MTL) preserve anterior and posterior hippocampal-cortical dissociations, but alter signaling between these and other key brain regions. During performance of a memory encoding task, we found reduced neural activity in human patients with unilateral temporal lobe epilepsy relative to age-matched healthy controls, but no upregulation of fMRI signal in unaffected hippocampal subregions. Instead, patients showed aberrant resting fMRI connectivity within anterior and posterior hippocampal-cortical networks, which was associated with memory decline, distinguishing memory-intact from memory-impaired patients. Our results highlight a critical role for intact hippocampo-cortical functional communication in memory and provide evidence that chronic injuryinduced functional reorganization in the diseased MTL is behavioral inefficient.

Differences in graph theory functional connectivity in left and right temporal lobe epilepsy

“…In addition, we show that edgewise connectivity of these structures are affected to different extents in left and right TLE, with the insula and parahippocampal gyrus most affected in right TLE, and the superior temporal gyrus and thalamus most affected in left TLE. The hippocampus and posterior cingulate were affected in both left and right TLE, similar to previous research (James et al, 2013), and is consistent with the selective vulnerability of functional connections between the hippocampal and posterior cingulate regions of the DMN identified in previous rs-fMRI studies (Voets et al, 2014). …”

“…This may reflect compensatory changes necessitated by greater cognitive deficits in left TLE (Mayeux et al, 1980), resulting in reorganization and less impairment from reductions in left TLE connectivity. Alternatively, this may also represent a maladaptive process leading to greater cognitive deficits in left TLE, as research increasingly supports the notion that upregulations in mesial temporal lobe connectivity are not functionally efficient (Voets et al, 2014). Further investigation is needed to improve understanding of circumstances under which these functional changes are compensatory or maladaptive (Saurand Hartwigsen, 2012; Voets et al, 2014).…”

Postoperative seizure freedom does not normalize altered connectivity in temporal lobe epilepsy

“…In other words, a plausible explanation for the significant differences in connectivity noted between the healthy and diseased state may be due to a disease-related reorganization of brain networks, or even merely because of individual variation, especially but not exclusively in terms spatial extent 35 . This is particularly relevant with regards to our conclusion that the changes in connectivity are “burnt in” after years of disease, given that reorganization of functional networks is likely to occur over longer time periods.…”

Evidence for an anterior–posterior differentiation in the human hippocampal formation revealed by meta-analytic parcellation of fMRI coordinate maps: Focus on the subiculum

“…Nevertheless, several fMRI studies have reported distinct patterns of activation across anterior and posterior regions of the hippocampus (e.g. (Baumann and Mattingley, 2013; Hirshhorn et al, 2012; Kuhn and Gallinat, 2014; Nadel et al, 2013; Strange et al, 1999; Voets et al, 2014). Another promising approach has been to examine patterns of resting functional connectivity with other structures, using for example, resting state fMRI (rsfMRI).…”