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Researchers have proposed that the ISF is generated from two possible sources:
The production of calcium waves induces oscillation in a type of glial cell called ‘astrocytes.’ This astrocytic oscillation triggers burst firing of the neurons, which causes frequencies to actively connect or synchronize and establish cross-frequency coupling.
Calcium waves create vasodilation and oxygen consumption, which results in the BOLD signal we measure in fMRI studies. This is believed to be why the ISF correlates with the BOLD signal.
The various networks – default mode, salience, central executive, and central autonomic – each have primary ‘hubs’ that are densely connected. The main hubs of these networks are collectively referred to as the ‘Rich Club.’ The ‘Rich Club’ has been shown to be bound by the infraslow fluctuation.
ISF is further believed to be both a foundational rhythm and a carrier frequency. Faster frequencies above the alpha band, especially beta and gamma, are informational frequencies. These are ‘nested’ in the infraslow fluctuations. Thus, when the infraslow rhythm fluctuates, faster frequencies change their behaviour in ways that directly correlate with what the ISF is doing. In this way, the ISF is an activating/deactivating signal. It is the control mechanism in the cortex that allows for the binding of faster frequencies and networks.
Following this reasoning, ISF is considered an ‘organizing’ rhythm; promoting fluid and timely brain function. In concept, when we train the ISF signal, we are supporting clients in their ability to globally regulate their systems. ISF, therefore, has a broad impact on function. This is why we expect to see a change in QEEG metrics from pre to post ISF training, despite the fact that we are not directly measuring the ISF in a QEEG.
New research reinforces that the organizational effects of the ISF are, in fact, acting on the autonomic nervous system. As a result of this association, training the ISF band helps to regulate the balance between sympathetic (fight, flight or freeze) and parasympathetic (rest, digest) activation.