If a high level of beta oscillations means that neurons involved in HFO are more constrained in their firing patterns, one could speculate that the level of PAC is a direct consequence of the amplitude of beta oscillations. Indeed, we found a highly significant positive correlation between the two. A complicating confounding factor in this discussion is the influence of signal-to-noise ratio, which may affect the accuracy of beta phase estimates and, in consequence, PAC. However, beta peak frequencies for individual power and PAC spectra did not overlap exactly. Also, permuting the order of trials for which phase time series were extracted, as typically done for non-parametric tests, destroyed the distinct patterns of significant PAC (results not shown), suggesting that it is not the level of beta power as such that leads to higher PAC estimates but its interaction with HFO.
