Previous studies on the effect of aging and AD on power and FC
Alpha FC has been shown to be more reproducible than other frequencies
(Duan et al., 2021), allowing high subject identifiability in resting
state MEG, especially in the visual subnetwork (Sareen et al. ,
2021). Consequently, the reduction in alpha FC with aging has been
demonstrated using different recording setups (MEG, EEG) and techniques.
For example, a recent MEG study showed that FC in Upper Alpha (UA) band
(10 – 14 Hz) falls with age (Pathak et al. , 2022), owing to the
reduction in individual peak alpha frequency (IAPF) with age (18 – 88
yrs), even though the FC (PLV and Phase lag index (PLI)) at the peak
alpha frequency remained invariant. This matches with the falling trend
of alpha (8 – 12 Hz) FC with age in our data (Figure 6a). Another study
in eyes-closed resting state EEG (Scally et al. , 2018) reported
reduced alpha power and FC in the conventional UA band (10 – 12 Hz) in
older adults compared to young adults, although power and FC at IAPF
were similar in the two age groups. Note that our results are not
inconsistent with previous studies that have shown reduction in alpha
power (Scally et al. , 2018) and IAPF with aging (Pathak et al.,
2022), since these studies used a much wider age distribution than ours.
We have previously shown that alpha power was indeed significantly less
in middle-aged subjects (50-64 years) compared to young subjects between
20-49 years (Murty et al. , 2020). Further, since we computed the
PSDs over 500 ms of data, the resulting frequency resolution of 2 Hz was
not enough to study changes in IAPF. Another study in resting state EEG
(Moezzi et al. , 2019) showed reduction in alpha FC (measured
using imCOH) within old adults compared to the young, along with gamma
FC, especially within the connections involving the occipital
electrodes.
With AD progression, Dai and colleagues showed prominent disruption in
FC in long-range connections (Dai et al. , 2015). These long range
connections across the brain have been proposed to play an important
role in aiding human cognitive function (Bullmore & Sporns, 2012). In a
MEG study, Berendse and colleagues showed reduced fronto-parietal FC in
2 to 22 Hz range in AD patients over controls (Berendse et al. ,
2000). FC measures like phase coherence and amplitude correlation have
been shown to provide insights into large-scale neuronal interactions,
which degrade in neuropsychiatric disorders (Siegel et al. ,
2012).
In addition, FC of low-frequency oscillations such as delta and theta
have also been shown to get altered with AD/ MCI. For example, a recent
study showed disrupted event-related EEG FC over intra-hemispheric and
midline theta band in AD subjects compared to MCI and healthy controls
(Fide et al. , 2022). Another study in resting EEG reported
reduced delta power in MCI patients (Liddell et al. , 2007) along
with ERP components such as N300 which is related to working memory.
Penttila and colleagues (1985) reported reduced theta power in
mid-temporal area and reduced alpha in the occipital area in subjects
with moderate AD over healthy controls. In addition, the study
highlighted the slowing of alpha rhythm, i.e., IAPF reduction in AD
patients. Olichney and colleagues (2002) showed that P600, an ERP
measure of episodic memory encoding, decreased in MCI patients.