Modeling the relationship between Higuchi’s fractal dimension and Fourier spectra of physiological signals
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The exact mathematical relationship between FFT spectrum and fractal dimension (FD) of an experimentally recorded signal is not known. In this work, we tried to calculate signal FD directly from its Fourier amplitudes. First, dependence of Higuchi’s FD of mathematical sinusoids on their individual frequencies was modeled with a two-parameter exponential function. Next, FD of a finite sum of sinusoids was found to be a weighted average of their FDs, weighting factors being their Fourier amplitudes raised to a fractal degree. Exponent dependence on frequency was modeled with exponential, power and logarithmic functions. A set of 280 EEG signals and Weierstrass functions were analyzed. Cross-validation was done within EEG signals and between them and Weierstrass functions. Exponential dependence of fractal exponents on frequency was found to be the most accurate. In this work, signal FD was for the first time expressed as a fractal weighted average of FD values of its Fourier components, also allowing researchers to perform direct estimation of signal fractal dimension from its FFT spectrum.
KeywordsFractal dimension FFT spectra EEG signals Weierstrass functions Higuchi’s method
This work was financed by the Ministry of Education and Science of the Republic of Serbia (Projects OI 173022 and III 41028). We thank Dr. Vlada Radivojević and Mr. Predrag Šuković for their help and cooperation in obtaining and analyzing the data and Dr. Žarko Martinović for second opinions in visual analysis and scoring of EEG records.
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