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Figure 2 Results (in normalized units, n.u.) of power spectrum analysis (PSA) of respiration and RR intervals (RR) during spontaneous and controlled breathing, reading (mentally or aloud), talking and mental stress (with or without verbalization). Note the inversion of the LF/HF ratio both in RR and respiration, mainly due to the increase in LF (white bars) as a consequence of slowing of breathing frequency or sympathetic activation (during mental stress), during reading aloud, free talking and mental stress, compared with spontaneous breathing. LF = low frequency, open bars; HF = high frequency, solid bars. *p < 0.05; **p < 0.01 versus spontaneous breathing; p < 0.05; p < 0.01; p < 0.001 versus silent reading; p < 0.05; p < 0.01; p < 0.001 versus controlled breathing.