The bicoherency (Nikias and Petropulu, 1993) is the normalized version of the cross-bispectrum, which is the analogous of the coherency for the cross-spectrum. The absolute value of bicoherency, i.e., the bicoherence, is a measure of the coupling between the phases in signals i and j at two possibly different frequencies, θi(f1) and θj(f2), with respect to the phase in signal k at a third frequency which is the sum of the other two, θk(f1 + f2), such that the mean resultant length of the phase difference Φijq(f1,f2)θi(f1)+θj(f2)-θk(f1+f2) is non-vanishing. Such a phenomenon is called quadratic phase coupling, and it conceptually different from the n:m coupling described above. There is one case in which the two phenomena coincide, that is the case of f1 = f2 =:f and f3 = 2f, in which the quadratic phase coupling involves only two frequency components, i.e., one frequency and its double, thus matching the 1:2 phase locking.
