[FieldTrip] Baseline Correction using 'relchange' or 'db'

[Herring, J.D. (Jim)]

Dear Mick,

Indeed, if you apply a baseline correction only while plotting the the TFR it could be that you get different results when running statistics on the non-baseline corrected data.

You can apply a baseline correction on your data prior to running the statistical analysis by using ft_freqbaseline to your ‘diff’ structure with cfg.baseline and cfg.baselinetype having the same values as you used while plotting.

Best,

Jim



On 29 Jul 2015, at 15:55, mick.lehmann at uzh.ch<mailto:mick.lehmann at uzh.ch> wrote:

Dear Fieltrippers,

yesterday I addressed this mailing list with an issue regarding the baseline correction for time frequency analysis. While plotting a comparison of two conditions revealed exactly the results I've expected, the statistic didn't at all. Regardless of electrode, time window and freq range, the comparison of the two conditions was significantly different (with t-values between 8 and 10 and p < 0.001). Digging a little deeper, the main difference between the script for plotting and the one for stat analysis is that in for the figure, I've to indicate again the baselinewindow and the baselinetype:


diff = gaTF_Cond1;
diff.powspctrm = (gaTF_Cond1.powspctrm - gaTF_Cond2.powspctrm);


cfg = [];
cfg.baseline     = [-1 -.1];
cfg.baselinetype = 'db';
cfg.maskstyle    = 'saturation';
cfg.zlim         = [-3 3];
cfg.ylim         = [2 25];
cfg.xlim         = [-1 2];
cfg.channel       = {'all'};
cfg.layout = 'C:\Users\MickLehmann\Documents\Emo_React_EEG\FieldTrip\Scripts\egi128_GSN_HydroCel3.sfp';
cfg.interactive = 'yes'
ft_multiplotTFR(cfg,diff);

Runningit like this, the figure looks fine. However, if I switch off the bold part, the figure is dark red (as the stat analysis would suggest).

In the statistic script, I don't indicate the baseline again and I've no clue how to implement it. Do you think that could be the problem and has anybody a suggestion how to address the problem?

If that is actually the main issue then I'm hardly the first encountering this problem.

Kind regards,
Mick
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