Time–frequency plots of the R–R interval. The upper four panels show the color-coded spectral densities as a function of time and frequency (black corresponds to zero value, red with variable intensity intermediate values, and white the maximum value). The methods used in analysis are, from left: fast Fourier transform (FFT), WVD, an autoregression (AR) model of order 12 (AR(12)), and an AR model of order 20 (AR(20)). The lower four panels display the local spectra as an intersection of the time–frequency plots marked with the vertical dashed white lines. The respiration frequency of 0.15 Hz is clearly visible on all plots. In the low-frequency (LF) range below 0.15 Hz, FFT and WVD methods can extract at least two dominant peaks at frequencies of 0.06 Hz and 0.09 Hz which are also visible as almost continuous paths on the time–frequency plots. These peaks can be distinguished with the AR(20) model but not with the AR(12) model.