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ISO-α denotes the nested oscillation between infra-slow oscillation (ISO) and the α oscillation, similar to the slow oscillation containing high-frequency oscillations (Figure 4-10). In principle, any astrocyte-coupled neuron containing HCN channels may display ISO-α. This has been demonstrated in slices isolated from the lateral geniculate nucleus (LGN) of the thalamus (Figure 5-7). The ISO-α is greatly facilitated by activating mGluR and/or mAChR (Lorincz et al, 2009). As explained in the previous section, activation of mGluR increases glutamate release from astrocytes, thereby enhancing the excitability of astrocyte-coupled neurons. Activation of mAChR elevates the excitability of HCN channel-containing neurons (Section 7.5). Therefore, activation of mGluR and/or mAChR can induce ISO-α.
The EEG exhibits robust α rhythms while a person is awake. During sleep, its power decreases in most cortical areas, except the frontal lobe. At the stage of deep sleep, the frontal α power becomes much greater than in wakefulness (Connemann et al., 2001). The effects of propofol (a commonly used anesthetic) on α activities are also region-dependent: enhanced in the frontal lobe while reduced in posterior parts (Vijayan et al., 2013). Other studies have found that the α waves originate from at least two different areas: thalamus and anterior cingulate cortex (ACC). The α waves originating from ACC propagate to the frontal lobe (Connemann et al., 2005), whereas the α waves originating from the thalamus propagate to posterior regions (occipital, temporal and parietal lobes) (Schreckenberger et al., 2004).
As described in Section 5.1, consciousness is closely related to the functional connectivity within resting state networks, especially between the frontal and posterior regions. The resting state networks exhibit synchronized ISO and the ISO can facilitate global synchronization. Since α is the dominant oscillation during wakefulness, the following hypothesis is proposed:
The Alpha Hypothesis:
This hypothesis is further supported by the observation that the ascending arousal system has two branches, one activating the thalamus and the other projecting to the frontal lobe (Section 7.3). The hypothesis can also explain the effects of general anesthetics on HCN channels and α rhythms (Section 7.8).