[FieldTrip] Why are time-frequency results so much different from Beamformer source localization?

Patrick Wiegel patrick.wiegel at sport.uni-freiburg.de
Fri Nov 13 11:20:11 CET 2020


Dear Field-trip community,

I am working on EEG data from a motor learning task. in the first part of my analysis, I performed a time-frequency analysis (Morlet wavelet method) and cluster-based permutation test. I compared two different conditions (correct & incorrect movements) within subjects and found a significant difference in the beta band (20-35 Hz) when subjects were provided with feedback. I attached the topographical plot from one subject that illustrates the difference between the conditions at the selected time point and frequency (dif_morlet). As you can see, beta power was higher in one condition in lateral frontal electrodes.

Now, I would like to better spatially locate the effect for this single subject. For this purpose, I followed the Beamformer tutorial on the website. In the first step, I selected the raw data of interest (at the time where the effect was seen) and performed frequency analyses for the 20-35 Hz range (mtmfft, contrast between conditions shown in dif_mtmfft). There is an apparent difference between dif_morlet and dif_mtmfft and I am wondering why this is the case? One apparent reason is that the time-frequency data were baseline corrected and the frequency data from mtmfft not.

Using the power and CSD data from mtmfft, I performed the source analysis with a standard MRI and head model. The source was located in the left hemisphere (source plot), which is in stark contrast to the time-frequency data (dif_morlet). I am aware that the source analysis is based on a lot of assumptions (conductivity etc.) that influence the calculations but it is very difficult to make sense of such discrepancy.
Maybe it is necessary to also baseline correct the mtmfft data before inputting them to the source analysis? Or doing the contrasting source localization betwee a single condition and the corresponding baseline data before contrasting the 2 conditions?

I am providing all necessary files and scripts (TF_analysis & BEAMFORMER) if anyone is interested to reproduce the results that I am showing and reporting for this single subject (https://github.com/PatrickWiegel/EEG-Beamformer-Source-localisation).

I would appreciate any help and discussion on that.

All the best,
Patrick





Patrick Wiegel
Department of Sport and Sport Science

University of Freiburg
Sandfangweg 4
79117 Freiburg i. Br .

phone: +49 (0)761/ 203-4550
email: patrick.wiegel at sport.uni-freiburg.de<mailto:patrick.wiegel at sport.uni-freiburg.de>
web: www.sport.uni-freiburg.de

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