Frequency Contours
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SAP2011 calculates frequency contours by detecting zero crossings of the spectral derivatives. In order to reject artifact, we require that the contours passed a dynamic contrast threshold, T, calculated for each time window t_{i} and frequency f_{i} as: T(t_{i},f_{i})=abs(Wiener_entropy(t_{i})) / abs(f_{i}mean_frequency(t_{i})), were T’ is a user defined threshold. Therefore, the detection threshold is weighted by the distance from the mean frequency (the gravity center of the frequencies) and by the width of the power spectrum. A pixel in the time frequency space is defined as contour if i) there is zero crossing between the neighboring pixels at any one of the 8 possible directions (see diagram below) and ii) both neighboring pixels (in the direction of the zero crossing) are larger than T. Here is an example with T'=10:
and with T'=50:
