Some study limitations are acknowledged, including the small number of electrodes used for EEG recording and data analyses that may affect the source localization results. However, the good localization property of the LORETA tomography has been evidenced in several studies applying 19-channel EEG systems. These studies have successfully localized functional abnormalities in cognitive disorders [33], [65] and correlated LORETA three-dimensional current source density with anatomical modules of synaptic activity measured by diffusion spectral imaging [81]. Since our analyses focused on functional connectivity in the 1–30 Hz frequencies, we cannot rule out that APOE-related lagged phase synchronization abnormalities during rest may appear within certain range of the gamma band. Further neurophysiological studies using high-channel EEG/MEG systems may shed some light on the role of resting-state gamma connectivity in dementia. Another potential limitation to be considered is that an APOE-4 homozygous group of AD patients was not included, due to the small number of ε4 carriers with two alleles (n = 5). Therefore, functional connectivity abnormalities possibly related to the dose of the APOE-4 allele (homozygous vs. heterozygous) could not be explored.
