Neural mechanisms underlying changes in power and FC
Reduction in alpha FC has been attributed to various changes in brain
morphology, like white matter hyperintensities (Quandt et al. ,
2020), cerebral reorganization or dedifferentiation processes (Eddeet al. , 2021). Recently, Ranasinghe and colleagues measured the
PSDs of AD subjects and healthy controls and found enhanced delta/theta
but reduced alpha/beta power, which they fitted using a neural mass
model by appropriately changing the excitatory and inhibitory
time-constants (Ranasinghe et al. , 2022). They also measured
regional tau and Aβ in AD patients by positron emission tomography.
Surprisingly, excitatory time-constants were tightly correlated with tau
levels, while inhibitory time-constants tightly correlated with Aβ
depositions.
The excitatory-inhibitory interactions can also produce gamma
oscillations (Wang, 2010). While fast gamma oscillations are thought to
be due to parvalbumin-positive GABAergic interneurons (Bartos et
al. , 2007), slow gamma could be related to long-range inhibitory
somatostatin interneurons, as reported in a mouse V1 study (Veitet al. , 2017). The GABAergic system gets altered with AD
pathogenesis, giving rise to E/I imbalance (Petrache et al. ,
2019). Indeed, cognitive functions in AD patients and mouse models can
be improved by the correction of E/I imbalance though drug intervention
(Bi et al. , 2020). Dysfunction of these inter-neuronal networks
with mental disorders could lead to lower gamma power and FC across
areas (Uhlhaas & Singer, 2006).
The mechanisms discussed above are likely to reduce both power and FC in
different frequency bands. How can then a reduction in alpha FC without
changes in power or vice versa be explained? One possibility could be
due to the presence of a variety of neuro-compensatory mechanisms that
may cause power and FC to change differently. For example, Pathak and
colleagues (2022) showed that the effect of increased axonal
transmission delays due to degeneration of white matter tracts with
aging can be cancelled by enhancing inter-areal coupling (Pathak et al.,
2022), leading to FC at IAPF that is invariant with age. Similarly,
Hinkley and colleagues (2011) reported unchanged alpha power but reduced
FC (measured through PLV) in Schizophrenia patients compared to
controls, which they attributed to disruption in long range connections
but intact local connections.