Fictitious heart rate signals and associated power spectra. The top panels represent pure high-frequency variability (0.25 Hz), as associated with RSA. The middle panels represent low-frequency variability (0.07 Hz), primarily of sympathetic and nonneural origin. the bottom panels represent the combined signal including both high- and low-frequency components. Actual heart rate signals include spectral power at additional frequencies (adapted from Mezzacappa et al., 1994).