Relationship between FC and power
We
found a negative correlation between PPC connectivity and change in
power in slow and fast gamma bands (Figure 6b). The reasons are unclear,
but in line with other studies that have reported similarities between
PSDs and network measures computed from FC measures such as PLV, and
cautioned against treating them as completely separate measures (Demuruet al. , 2020). One possibility is that when the oscillatory
signal is weak, the FC is governed by the background spectral power
which may have a larger spatial spread. Another possibility is that the
source of the oscillation is deeper, which would lead to lower power at
the sensor level but nonetheless stronger FC across sensors since more
sensors would capture the source activity.
On the other hand, several reports have shown that power and FC capture
different features of the underlying neural activity. For example,
differences in power versus FC with age and cognitive decline (MCI)
reported here have also been observed in resting state EEG, although the
strongest effect is observed at lower frequencies such as delta and
theta bands (Meghdadi et al. , 2021). Similarly, FC could convey
different information compared to PSDs about mental disorders. For
example, patients with schizophrenia have higher EEG complexity at lower
frequencies (Takahashi et al. , 2010), which is shown to reflect
reduced FC (Friston, 1996). In addition, FC is shown to capture subject
specific information like the demographic variables, while being
consistent across a variety of tasks (Nentwich et al. , 2020).
However, a recent study reported source and sensor space FC to be less
reproducible than either the absolute or relative powers (Duan et
al. , 2021). Note that most of these earlier studies used resting state
EEG data. Our study extends these findings for stimulus-induced
oscillations and shows that power and FC alter in distinct ways with
aging and cognitive disorders, and the changes in FC can be observed
even after accounting for the difference in power. Indeed, some studies
have shown that compared to resting-state EEG, a paradigm involving
visual stimulation/perception is more sensitive to AD disease effects
(Barzegaran et al. , 2016).