njit-etd2003-081 - New Jersey Institute of Technology

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The time-frequency plots using the Monet wavelet is shown in figure 5.10.
Figure 5.10 (a) shows the 3D plot in time (seconds), frequency (Hertz) and wavelet
coefficient (power). Figure 5.10 (b) and (c) are contour plots of figure 5.10 (a) looking
along the z-axis (wavelet coefficients, power). Figure 5.10 (b) shows a relatively straight
band at the 1024 scale, which corresponds, to 0.133 Hz due to paced breathing at 8 bpm.
Furthermore, the plot also shows another band of high amplitude at 128-256 scales, which
corresponds to 0.635-1.27 Hz; the same is also shown on all other time-frequency plots.
The meaning of this band may possibly be due to the 1 Hz cyclic modulation of the ECG
rhythm that is out of our frequency range of interest. When zooming into a closer
frequency range as illustrated in figure 5.10 (c) the 0.1333 Hz band is not a straight band
but varies in frequencies from 0.09 Hz and up to 0.2 Hz. This information exists on all
time-frequency plots except it is much better represented on all plots using the wavelet
transform. This band is also not solid but made up of many lobes called the cones of
influence. Each cone of influence occurs at the minimum and maximum points of the
original HR 11131 signal being analyzed which is shown in figure 5.10 (d) again for ease of
observation. Figure 5.10 (c) also shows another band of cones of influence along the
lower frequency range of 0.01 — 0.05 Hz that mimics the trend of the lower frequency
modulation present in the HR IIBI signal. This is additional information about the trend
of HRV signals, which is shown for the first time using wavelet analysis. This
information is totally absent when using the Cohen's class time-frequency representations
such as SPWD, CWD or BJCD.