The main spectral peaks of the R–R interval. The R–R interval (RRI) time series of 12 subjects were spectrally decomposed by computing the AR spectrum and searching the frequency components corresponding to the poles of the spectral density function. The results in the LF range are shown as a function of time and frequency. The color scale displays the spectral power of each component (black corresponds to zero value, red with variable intensity intermediate values, and white the maximum value). The natural breathing frequency of each subject was different, and is marked with a red dot. Case A is the same as shown in the Figure 1.