[FieldTrip] Strange behavior of mtmconvol

[Martin Schaefer]

Dear fieldtrip community,

I'm getting strange results when I compute the powerspectrum of my data with the mtmconvol method.
When I plot my results over time and frequencies, I get most power in the higher frequencies, compared to the wavelet, tfr, or superlet methods where I get most power in the lower frequencies.
When I look at just the power over frequencies, it looks like mtmconvol removes 1/f. Is this indeed the case? If so, why does it do this?
I attached plots of the powerspectrum for the mtmconvol method, superlet method, and superlet method adjusted for 1/f by multiplying the power with the frequency which makes it look a lot more like the mtmconvol results.

My settings:
fieldtrip-20220401

cfg                    = [];
cfg.output          = 'pow';
cfg.keeptrials      = 'yes';
cfg.channel         = 'all';
cfg.method         = 'mtmconvol'; /'superlet'
cfg.taper             = 'dpss';
cfg.foi                 = 4:.5:100;
cfg.t_ftimwin       = 7./cfg.foi;
cfg.toi                = 'all';
cfg.polyremoval    = 1;
cfg.pad               = 11;
cfg.tapsmofrq      = 0.3.*cfg.foi ;

% for 'superlet' method
cfg.width            = 3;
cfg.gwidth          = 3;
cfg.combine         = 'additive';
cfg.order            = round(linspace(1,40,numel(cfg.foi)));

ft_freqanalysis(cfg, data);

I'm grateful for any input.

Best regards,
Martin



------------

Martin Schaefer | PhD Student

https://www.perceptionlab.se/

Department of Clinical Neuroscience (CNS), K8

Karolinska Institutet | 17 177 Stockholm | Nobels väg 9

martin.schaefer at ki.se<mailto:artin.arshamian at ki.se>

______________________________________
Karolinska Institutet – a Medical University




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