# fourierspectra and freqstatistics

Jasper Poort j.poort at NIN.KNAW.NL
Wed Aug 5 10:43:29 CEST 2009

Dear all, I have the following question

I want to compare the powerspectra in two conditions for individual
channels.

When I first run freqanalyis with

cfg.output      = 'pow';

cfg.method      = 'mtmconvol';

cfg.keeptrials  = 'yes';

this results in a power spectrum for two conditions:

size(A.powspctrm): 943     1    19    48 ntrials x nchannels x nfrequency x
ntime

size(A.cumtapcnt): 943    19

I can then compare the powerspectra in two conditions by running
freqstatistics with:

cfg = [];

cfg.method    = 'analytic';

cfg.statistic = 'indepsamplesT';

ntrl1 = size(A.powspctrm,1);

ntrl2 = size(B.powspctrm,1);

design = zeros(1,ntrl1 + ntrl2);

design(1,1:ntrl1) = 1;

design(1,(ntrl1+1):(ntrl1+ntrl2)) = 2;

cfg.design      = design;

cfg.ivar        = 1; % independent variable

stat = freqstatistics(cfg,A, B);

differencepowspctrm = stat.stat;

However, when I use freqanalyis with

cfg.output      = 'fourier';

this results in a fourier spectrum for two conditions:

size(A.fourierspctrm):  4715    1          19          48 (ntapers (in my
case 5) x ntrials) x nchannels x nfrequency x ntime

size(A.cumtapcnt)    :  943     1

Can I input these fourier spectra directly to freqstatistics or should I
convert the fourierspectrum first to a powerspectrum (I assume this could be
done by taking abs(fourierspectrum).^2 and then averaging over tapers for
every trial ) ?

Alternatively, is it at all possible to first run freqdescriptives and then
input the averages and  variance into freqstatistics (since this is the only
thing necessary for the independent t-test)?

Many thanks in advance, best, Jasper

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