Finally, we investigated whether the amplitude of HFO in the STN was also locked to the phase of beta oscillations in the motor cortex. The grand average across all recordings is shown in Fig. 6(A). In general, PAC between STN and cortical motor areas was found to be weaker compared to PAC within STN and distinct patterns within the spectrum were only rarely observed. For example, we only found 1 case in which a cluster of >400 adjacent significant points of beta/HFO frequency combinations could be detected, compared to 13 cases for PAC within STN for this subgroup of subjects. A possible explanation for the lack of clear STN-cortex PAC emerges when we directly compare the within-STN PAC and the STN-cortex beta coherence (Fig. 6B). This reveals that PAC within the STN mainly occurs for lower beta band frequencies (∼17 Hz), whereas STN-cortex coherence is dominated by higher beta band frequencies (∼25 Hz). No significant correlations were found between the UPDRS scores and average STN-cortex PAC in the low-beta or high-beta band range, nor for cortical power or coherence.
