Figure 4: Surrogate data test. | Nature Communications

Figure 4: Surrogate data test.

From: In vivo cardiac phase response curve elucidates human respiratory heart rate variability

Figure 4

Here we show an essential difference between true functions Qe (here for ECG-respiration) and spurious functions Qs. (a,b) Coupling functions for two recordings of the same subject (average heart rates are 0.93 Hz and 0.94 Hz, average respiration frequencies are 0.16 Hz and 0.13 Hz). (c,d) Coupling functions obtained after interchanging the respiratory signals from these two measurements. (e,f) Here respiration time series of a different subjects is used (average frequency 0.32). The difference measure, η, between the true functions (a) and (b) is 0.20, whereas the difference between the true function (a) and the spurious functions (c,e) is 0.81. The difference between (b) and its surrogates (d,f) is 0.80. The difference between the surrogates is in the range from 0.58 to 0.80. The correlation, ρ, between the true functions (a) and (b) is 0.93, whereas the correlation between the true function (a) and the spurious functions (c,e) is −0.10 and 0.00, respectively. The correlation between (b) and its surrogates (d,f) is 0.13 and 0.25. The absolute value of the correlation between the surrogates is in the range from 0.02 to 0.34.

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