Phase-amplitude coupling was calculated using a method that has been previously described.24 Briefly, potentials recorded in ECoG were first bandpass filtered into a low frequency band (6–50 Hz in a 2 Hz step, without overlap) and a high frequency band (50–200 Hz in a 4 Hz step, without overlap) using a two-way zero phase lag finite impulse response filter. Then, the instantaneous phase of the low frequency bandpass filtered signal and the instantaneous amplitude of the high frequency filtered signal were extracted through the Hilbert transform. The modulation index (MI) was derived using the Kullback-Leibler distance that measures the divergence between the probability distribution of high-frequency amplitudes and uniform distribution. The obtained MI was normalized by calculating the z-score of 200 surrogates generated by randomly swapping amplitudes time blocks.25Z-scored PAC computed for multiple frequencies of phase and amplitude can be demonstrated as a comodulogram (Fig. 2A). We used the Tensorpac toolbox (https://etiennecmb.github.io/tensorpac/)26 to conduct all PAC calculations.
