Fig. 1(C) shows the profile of PAC across beta band and HFO frequencies separately, i.e. when averaged across all amplitude frequencies or phase frequencies. There is a striking correspondence between these curves and those for the power spectral densities in the corresponding frequency ranges (Fig. 1D). In particular, there was a spectral power peak in the low beta range that corresponded to the peak frequency of phase modulation of HFO in the OFF medication state. This low beta activity in power and PAC was significantly suppressed by medication. By contrast, the grand-average spectral density demonstrated a second peak in the upper beta band, which was not associated with phase modulation of HFO and was less responsive to medication. At higher frequencies the grand-average spectral densities demonstrated a peak centred at 250 Hz OFF medication, which shifted to 320 Hz ON medication. This was roughly paralleled by changes in the frequency of phase modulated HFO. Similar shifts in peak frequency have been observed by (López-Azcárate et al., 2010, Özkurt et al., 2011).
