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.