If the true signals sj(t) are uncorrelated, then the above reduces to Sxy (f) =
= Y1 aXaySSiSi (f), which is real valued as a result of the symmetry of the autocorrelation. A nonzero imaginary part in the cross spectrum must be due to something other than linear crosstalk and can be interpreted as a "true" interaction. Consequently, by considering the imaginary part only, or "coherency", one can eliminate the effect of crosstalk. While this approach can be used to detect interactions, quantitation is difficult since the real part of the cross spectrum, which contains information about both linear mixing and true interactions, is discarded.
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