[FieldTrip] ft_freqstatistics for between subject design

Beom Jun Min mbj0310 at gmail.com
Wed Mar 19 11:10:08 CET 2014

Hello trippers.

I am trying to do* t-test* with *the power* derived from frequency analysis
of two different groups.
The first group has 36 subjects, and the second group has 28 subjects.
I did the ft_freqanalysis with the Morlet wavelet, and made the* induced
power* of gamma band.

After the plotting of the results, it looks like there might be a
difference in the frequency range between [38 42], at the latency between
[0.5 1] approximately.

So, I made a script as below.

cfg = [];
cfg.channel     = 'PZ'; % name of the electrode
cfg.latency     = [0.5 1];
cfg.trials = 'all';
cfg.frequency = [38 42];
cfg.avgoverchan = 'no';
cfg.avgovertime = 'yes';
cfg.avgoverfreq = 'no';
cfg.parameter   = 'powspctrm';

cfg.method      = 'analytic';
cfg.statistic   = 'indepsamplesT';
cfg.alpha       = 0.05;

cfg.design = [ones(1,36), ones(1,28)*2] ; % N x num of observations
cfg.ivar = 1;

stat = ft_freqstatistics(cfg, Induced_Ctrl_14{1:36}, Induced_CHR_14{1:28});

In terms of the structure of the result, there are five values in the stat,
prob, mask and freq.
I think that each 5 values correspond with the frequency data point.

Is that script suitable for the test for the significance, especially
avgovertime option 'yes'?

If I change the avgovertime option to 'no', then can I found the p-values
for each frequencies(5 values), and each time point(51 values), in total
255 values that could have multiple comparison issue?

And I thought the 'mask' values of 0 indicates 'statistically
non-significance'. Is that right?

BeomJun Min, M.D.

Department of Medical System Engineering (DMSE)
Gwangju Institute of Science and Technology (GIST)
261 Cheomdan-gwagiro(Oryong-dong), Buk-gu, Gwangju
500-712, Republic of Korea (South)
Phone: +82-62-715-3266 / Fax: +82-62-715-3244
E-mail: mbj0310 at gmail.com, http://bmssa.gist.ac.kr
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