Importantly, high frequency power can be modulated by the phase of multiple slow brain rhythms simultaneously. In particular, Voytek and colleagues showed that the theta (4–8 Hz) rhythm dominated phase-amplitude CFC with gamma (30–80 Hz) band activity in anterior frontal and temporal sites during an auditory task (Figure 1C), while the strongest CFC over occipital areas during a visual task depended on the phase of the alpha (8–12 Hz) rhythm (Figure 1D) [55]. Gamma power was phase-locked to both theta and alpha phase, but the relative influence of each low-frequency rhythm varied as a function of brain area and task modality. Given the functional association of alpha band activity with visual cortical areas and processing, alpha-based phase-amplitude CFC may play a role in computation within visual cortical areas.
