The mutual coherence and phase of the RRI and SAP. An example of the FFT-based mutual coherence and the phase difference of RRI and SAP as a function of frequency. Both functions were computed using the same sliding windows (width 180 seconds, step 5 seconds) as used in analysis of Fig. 1, and the results are superimposed. The principal frequencies below the respiration frequency are marked with red horizontal bars and the respiration related frequency with a blue bar.