Regional analysis of differences in clustering coefficients as a function of correlation thresholds showed that the left and the right hippocampal regions differed significantly between groups. In contrast, the clustering coefficient of the precentral gyrus did not differ between groups. This suggests disrupted connectivity from the hippocampus to other regions of the brain in AD. This finding is consistent with our previous study [10] showing that AD reduced functional connectivity of the hippocampus within a specific network of regions—the default mode network [27],[28] that includes the posterior cingulate and lateral temporoparietal cortices. It is also consistent with the study by Wang et al. [16] showing altered hippocampal connectivity to several neocortical regions in the early stages of AD. Other studies have reported decreased intrahippocampal synchrony of low frequency BOLD fluctuations [17] during a task-free scan. Taken together, these findings point to significantly altered local and global hippocampal network connectivity in AD.
