Given that characterizing brain activity purely in terms of anatomically segregated responses is not sufficient to explain the complexity of AD symptomatology, recent studies have focused their attention on functional connectivity between brain regions using functional Magnetic Resonance Imaging (fMRI) [10]–[16], electroencephalography (EEG) [17]–[19] or magnetoencephalography (MEG) [20]–[22]. Functional connectivity refers to the temporal synchrony or correlation between signals of two or more spatially separated regions as an index of functional integration between neural populations [23]–[25]. Findings from most neuroimaging and neurophysiological studies proposed that disrupted functional connectivity represents a core pathophysiological mechanism underlying AD. This is supported by reports of white matter structural abnormalities in AD, using diffusion tensor imaging or MRI tractography, as measures of anatomical connectivity [26]–[28]. This evidence highlights the importance of regarding AD as a functional and structural network disorder.
