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Figure 3 Power spectra of RR interval (RR) and respiration obtained in one subject during spontaneous breathing, free talking and mental stress aloud and silently. During free talking and mental stress aloud, the frequency of breathing decreases, but during mental stress aloud this fact cannot be totally responsible for the increase in the low frequency components in RR because of the evident decrease in respiratory power (i.e., decrease in tidal volume) and the lack of exact coincidence between the low frequency peaks in respiration and RR. Conversely, the increase in the low frequency components in RR during free talking can be explained, to a great extent, by the slowing of respiratory frequency (no decrease in respiratory power and tidal volume). a.u. = arbitrary units; ms = milliseconds.