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Figure 1 Effects of spontaneous and controlled breathing, reading (mentally or aloud), talking and mental stress (with or without verbalization) on respiratory frequency and on relative changes in minute ventilation (100% corresponds to minute ventilation during spontaneous breathing). Note that, in conditions associated with verbalization or mental stress, respiration is slowed down and falls into the LF band of RR interval (0.03–0.15 Hz). HF = high frequency; LF = low frequency. *p < 0.05; **p < 0.01; ***p < 0.001 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; #p < 0.05; ##p < 0.01 versus mental stress aloud.