[FieldTrip] Different methods in calculating the amplitude modulation of a specific frequency yield different results

[christian.merkel at med.ovgu.de]

Hi to the fieldtrip community,


Currently I am browsing through all the different methods for analysing SSVEP data. Especially I am interested to look at the temporal modulation of a signal of a certain frequency.

There is a variety of methods to get that modulation over time and I noticed that there are some considerable differences:


(1) Up until now I used a (narrow-) bandpass-filter (usually with a 'fir'-filter) on the initial signal and applied the hilbert-transform to get the analytical signal.

(2) ft_freqanalysis provides me with another method 'tfr' that convolves my initial signal with a wavelet comprised of the frequency of interest.

 - both those methods yield quite similar results.


(3) However some groups in the ssvep-literature perform 'complex demodulation' which in its implementation in MATLAB includes a lowpass-filtering step (butterworth) as well. Using the MATLAB-function 'demod' yields quite dissimilar results to the previous two methods.


Is there any implementation for the complex demodulation of a signal included in fieldtrip? And how does that algorithm give such a different result compared to the filter/hilbert and wavelet convolution?

I attached three figures to illustrate my point. Each is the modulation of 14.4Hz of the same signal but calculated with 3 different methods.



Mit freundlichen Grüßen

Christian Merkel

<http://www.med.uni-magdeburg.de>
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