The different oscillations generated in cortical networks show a hierarchical relationship, often expressed by cross-frequency phase modulation between the various rhythms. This term implies that the phase of a slow oscillation modulates the amplitude of a faster rhythm, meaning that its amplitude varies predictably within each cycle. In turn, the phase of the faster rhythm modulates the amplitude of an even faster one and so on. This hierarchical mechanism is not unique to the brain. Spring, summer, fall, and winter are four phases of a year that “modulate” both the amplitude and duration of
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