source coherence in one condition
Andrew Smart
andrew.smart at NYU.EDU
Tue May 8 03:36:01 CEST 2007
Hi Robert,
Thank you very much for the code and the answers. It helps to clarify things alot. My understanding of coherence was that if the phase difference between two signals is confined to within 2pi over some time interval, they are phase locked, ie the temporal variations in the signals tend to follow each other. But that this is regardless of their differences in amplitudes. When amplitude also covaries then there is 'true' coherence. I thought that you could test for signifant coherence between two signals.
For example in BESA you can do permutation tests on individual subjects for 'significant' coherence between a reference dipole and the rest of the dipoles in a model in one condition. I might be misunderstanding something though becuase I was wondering if one could do this on a group level by importing the coherence data from BESA into fieldtrip. I suppose this would be coherence relative to some baseline, but I am not sure if that makes any sense. I don't know exactly how BESA calculates coherence.
So I guess H0 would be that there is no coherence between two dipoles that is not due to chance in a particular condition - is that a possible null hypothesis for coherence?
Thank you again for all your help!
andy
----- Original Message -----
From: Robert Oostenveld <r.oostenveld at FCDONDERS.RU.NL>
Date: Monday, May 7, 2007 3:12 pm
Subject: Re: [FIELDTRIP] source coherence in one condition
To: FIELDTRIP at NIC.SURFNET.NL
> On 26 Apr 2007, at 0:35, Andrew Smart wrote:
>
> > I would like to try to compute whether there is significant
> > coherence across
> > subjects but within one condition and between two dipoles. This code
> > generates graphs but I am unclear about what they mean:
>
> Hi Andrew,
>
> It is unclear to me what the comparison is in the statistical test
> that you want to perform. There should be a hypothesis like
> "coherence is larger in XXX than YYY", which rephrased would be H0:
>
> the coherence is the same. Based on the H0, the probablility can be
>
> computed and H0 can (perhaps) be rejected. You cannot say that
> "coherence is significant" just like that.
>
> If the hypotheis is that coherence between one dipole pair is larger
>
> than between the other dipole pair, then I am affraid that you cannot
>
> test that using a permutation approach. With a permutation or
> randomizarion test you test the hypothesis that the data in two
> observed conditions is exchangeable. Simple differences in power in
>
> one of the 4 sources underlying the two dipole pairs would already be
>
> enough to reject the null-hypothesis that the data is exchangeable,
>
> but those power differences do not have to mean that there is a
> coherence difference.
>
> Robert
>
> ----------------------------------
> The aim of this list is to facilitate the discussion between users of
> the FieldTrip toolbox, to share experiences and to discuss new ideas
> for MEG and EEG analysis. See also http://www.ru.nl/fcdonders/fieldtrip.
>
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