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

mick.lehmann at uzh.ch mick.lehmann at uzh.ch
Mon Jul 27 18:24:03 CEST 2015

Dear Fieldtrippers,

I did the baseline correction using the following script:

    TF_Remem_low = ft_freqbaseline(cfg, TF_Remem)
    cfg.baseline     = [-1 -.1 ];
    cfg.baselinetype = 'relchange'; HERE, I'VE TRIED 'DB' AND 'RELCHANGE', THE OUTCOME DIDN'T DIFFER
    cfg.parameter    = 'powspctrm';

This was done after ft_freqanalysis and before calculating the grand average. 

During the trial definition, I've already specified the baseline window and used 'demean':

	cfg.demean = 'yes';
	cfg.baselinewindow = [-1 -.1];

When I made the figure for a contrast, it looked fine and exactly as I expected (see attachment). However, calculating the statistics using the cluster-based permutation test over a time window of 0-2000ms and between 5 and 7 Hz (without averaging for time and frequency range), the comparison between the two conditions for every single time and frequency bin was highly significant (p< 0.001). This observation is not congruent with the figure, especially the fact that the t-values are almost equal across the 2 seconds and across the freq range.

Does anybody have an idea what might be the problem? Are the provided details sufficient for understanding the procedure?

Here the script for the cluster-based permutation:

cfg = [];
cfg.channel          = 'all';
cfg.latency          = [0 2];
cfg.avgovertime      = 'no';
cfg.frequency        = [5 7];
cfg.avgoverfreq      = 'no';
cfg.method           = 'montecarlo'; 
cfg.statistic        = 'ft_statfun_depsamplesT';
cfg.correctm         = 'cluster';
cfg.clusteralpha     = 0.001;
cfg.clusterstatistic = 'maxsum';
cfg.minnbchan        = 2;
cfg.tail             = 0;
cfg.clustertail      = 0;
cfg.alpha            = 0.001;
cfg.numrandomization = 1000;
cfg.neighbours       = neighbours;

I'm deeply grateful for every comment.
Best regards,
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