[FieldTrip] Change in output of ft_freqanalysis

[Harris, Alison]

Dear FieldTrip community,

My name is Alison Harris and I have been using FieldTrip functions for time-frequency analysis for several years now as part of my EEG research. However, I recently attempted to re-analyze some old data using ft_freqanalysis in the latest version of FieldTrip, and the output does not match my earlier results.

I am trying to perform a frequency analysis for a motor execution task with alternating 8-s periods of finger-tapping and rest. The two conditions are preprocessed in EEGLAB and then separately imported to FieldTrip for frequency analysis. The cfg and command are as follows:

    freqcfg = [];
    freqcfg.method = 'mtmfft';
    freqcfg.output = 'pow';
    freqcfg.keeptrials       = 'no';
    freqcfg.keeptapers       = 'no';
    freqcfg.foi = [1:30];
    freqcfg.taper = 'dpss';
    freqcfg.tapsmofrq  = 1;
    freqcfg.pad              = 'nextpow2';
    freqcfg.feedback         = 'text';
    [freqdata] = ft_freqanalysis(freqcfg,tldata);

When I initially ran this analysis in 2019, the power spectra for action and rest conditions were largely overlapping. However, when I reran the analysis earlier this week (using the same data and cfg), the two power spectra were offset, such that one power spectrum was noticeably lower on the y-axis than the other. For the two participants I examined, there was not a consistent pattern in terms of which power spectrum was offset: the power spectrum for the finger-tapping condition was higher in one, whereas the resting power spectrum was lower in the other.

I then tested out my program with two earlier versions of FieldTrip that were available for download: fieldtrip-20191231 and fieldtrip-20191213. The change in the output of ft_freqanalysis seems to date to the 2019 version, as the 2018 output looked more similar to my existing results from a previous version of FieldTrip. I also tried specifying the “polyremoval” parameter, and redid the preprocessing in EEGLAB with and without detrending, but none of these changes seem to fundamentally affect the offset of the power spectra.

Here is a link with data from two participants and plots of the power spectra from before and after 2019: https://urldefense.com/v3/__https://claremontmckenna.box.com/s/gu796rvvsgipnwkm4cb13k5fhtdpu5f0__;!!HJOPV4FYYWzcc1jazlU!_AWV1fOSVX5tHme3OeHf6q76srFJ4TcvvLtenRnbF2VDh6fOQW4CMFSLoytvKYc-XQQjU18ERSfYG-nv1jeraFpKi4vjyHuaNupSNy8$ .

Is there some difference in how the ft_freqanalysis command is called from 2019 onward that would explain this output?

Thank you for your help!
Alison Harris

--
Alison Harris, PhD
Ostby-Cook Associate Professor of Psychology & George R. Roberts Fellow
Claremont McKenna College
850 Columbia Avenue
Claremont, CA 91711
Phone: (909) 607-9397
Email: aharris at cmc.edu<mailto:aharris at cmc.edu>
Web: https://urldefense.com/v3/__https://www1.cmc.edu/pages/faculty/aharris__;!!HJOPV4FYYWzcc1jazlU!_AWV1fOSVX5tHme3OeHf6q76srFJ4TcvvLtenRnbF2VDh6fOQW4CMFSLoytvKYc-XQQjU18ERSfYG-nv1jeraFpKi4vjyHua46XzWYw$ 



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