Aging and Alzheimer’s Disease Have Dissociable Effects on Local and Regional Medial Temporal Lobe Connectivity

Abstract

AbstractBackgroundThe extent to which pathological processes in aging and Alzheimer’s disease (AD) relate to functional disruption of the medial temporal lobe (MTL)‐dependent brain networks is poorly understood. To address this knowledge gap, we examined functional connectivity (FC) alterations between anterior and posterior regions of the MTL and in MTL‐associated functional communities – the Anterior‐Temporal (AT) and Posterior‐Medial (PM) networks – in normal agers, individuals with preclinical AD, and patients with Mild Cognitive Impairment or mild dementia due to AD.MethodIn this cross‐sectional study, we analyzed data from 179 individuals from the Aging Brain Cohort study of the Penn ADRC. Detailed information about participants is provided in Table 1. For intra‐MTL FC comparisons, the MTL subregions were segmented using the automated segmentation of hippocampal subfields‐T1 (ASHS‐T1) pipeline (Fig. 1a). When modeling the MTL’s interactions with the rest of the cortex, we employed four MTL ROIs (left/right × anterior/posterior) derived from an ex vivo atlas of tau accumulation in the MTL. Our functional datasets were preprocessed using a customized fMRIprep pipeline. Sparse network estimations and modularity‐based consensus clustering were used to reconstruct the AT and PM network systems (Fig. 1b). Age effect analyses and group comparisons along the AD continuum were performed using the General Linear Model within the network‐based statistical framework.ResultsThe preclinical stage of AD was characterized by increased FC between the perirhinal cortex and other regions of the MTL, as well as between the anterior MTL and its direct neighbors in the AT network (Fig. 1c‐d). This effect was not present in symptomatic AD. Instead, symptomatic patients displayed reduced hippocampal and intra‐PM connectivity. For normal aging, our results led to three main conclusions (for visuals, see Fig. 2). First, intra‐network connectivity of both the AT and PM networks decreases with age. Second, FC between the anterior and posterior segments of the MTL declines with age. Finally, within the MTL, we observed greater vulnerability of the posterior MTL subregions, particularly the parahippocampal cortex, to age‐associated FC decline.ConclusionTogether, the current results provide evidence for aberrant connectivity in the preclinical stage of AD that may have implications for early AD pathophysiology.