[Eeglablist] A tentative issue in coherence calculation in EEGLAB for epoched data

Makoto Miyakoshi mmiyakoshi at ucsd.edu
Thu Jul 23 15:43:33 PDT 2015

Dear Iman,

I ask you this without testing it, but does the order of the averaging
process makes difference in results?

Makoto

>
>
> Hi EEGLABERs,
>
> I have been looking into ‘newcrossf’ function and the way it calculates
> coherence for epoched data. Basically, it uses the ‘timefreq’ function to
> calculate the time/frequency decomposition the data.  ‘timefreq’ function
> treats the epoched data as a continuous one:
>
>
>
> X = reshape(X, g.frame, g.trials);
>
> [alltfX freqs timesout] = timefreq(X, g.srate, spectraloptions{:});
>
>
>
> Y = reshape(Y, g.frame, g.trials);
>
> [alltfY] = timefreq(Y, g.srate, spectraloptions{:});
>
>
>
> and calculates the its spectrum using the whole data which is now
> concatenated version of all trials.  So, for each of channel’s pair (X and
> Y , for example) the spectrum is calculated as described above and then the
> joint time-freq decomposition is calculated for coherence value.
>
>
>
> coherres = sum(alltfX .* conj(alltfY), 3) ./ sqrt( sum(abs(alltfX).^2,3)
> .* sum(abs(alltfY).^2,3) );
>
>
>
> *However, similar to the ERSP concept, each trial/epoch might be different
> than others [because of perturbations in subjects’ mental status, mental
> fatigue, etc] and thus I think it is more appropriate to calculate the
> coherence for each trial first and then make the average across trials.*
>
>
>
> Any thoughts?
>
> Iman
>
>
>
>
>

--
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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