[FieldTrip] Question on baselining methods (absolute vs db)

[Sebastian Sauppe]

Dear FieldTrip list members,

I’ve got a question on the different methods available in ft_freqbaseline.

I have TFA data from an experiment where participants saw pictures and reacted to them. There is a pre-trial period (-1000 to 0 ms) during which the fixate a fixation cross and a trial period (0 to 1200 ms) during which they see the picture. I used mtmconvol in ft_freqstatistics to get the power. The frequencies are 4 to 20 Hz and I used 3 cycles (cfg.t_ftimwin  = 3./cfg.foi). Then I want to baseline the data for plotting and analysis. As baseline period I chose -500 to -200 ms. (But using other baseline periods don’t change the picture.)

When I use an absolute baseline, the activity during the baseline period is around 0 and during the trial there are increases and decreases (i.e. positive and negative values). So this looks like one would expect.

When I use a dB baseline, however, the activity during the baseline period is not around 0, but rather around -1 to -2 dB. During the trial, I also get values around -1 to -3 dB. This is the command:

cfg = [];
cfg.baseline = [-0.5 -0.2]; % baseline period = -500 to -200 ms, relative to stimulus onset
cfg.baselinetype = 'db';
cfg.parameter = 'powspctrm';
 
freq_data_baselined = ft_freqbaseline(cfg, freq_data);

What could be the reason for this discrepancy between absolute and dB baselines?

Thanks a lot for your help already in advance!

Regards,
Sebastian

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Dr. Sebastian Sauppe
Department of Comparative Linguistics, University of Zurich
Homepage: https://sites.google.com/site/sauppes/ <https://sites.google.com/site/sauppes/>
Twitter: @SebastianSauppe <https://twitter.com/SebastianSauppe>
Google Scholar Citations: https://scholar.google.de/citations?user=wEtciKQAAAAJ <https://scholar.google.de/citations?user=wEtciKQAAAAJ> 
ResearchGate: http://www.researchgate.net/profile/Sebastian_Sauppe <http://www.researchgate.net/profile/Sebastian_Sauppe>
ORCID ID: http://orcid.org/0000-0001-8670-8197 <http://orcid.org/0000-0001-8670-8197>
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