t statistic of coherence difference between stimulus conditions

[jan-mathijs schoffelen]

Dear Dahlia,

Your question has been asked before, so it may be worthwile to browse
the archive of the mailing list a bit.
In short, the way to go depends on what you want:

1) Do you want to test for significance within a subject? Then it may
be worthwhile to have a look at Maris et al 2007 J.Neurosci.Methods.
Here we describe a way to test for significant coherence differences
using permutation tests.
2) If you want to test for significance across subjects (which I
generally prefer), I would compute the coherence difference per
subject and apply a Z-transformation to the difference. Subsequently I
do a permutation test across subjects. By randomly swapping the sign
of the Z-transformed difference you obtain a distribution under the
null-hypothesis, i.e. no difference in coherence across subjects. (for
a formulation of the z-statistic you can have a look at the Maris
paper).

Both options are possible in fieldtrip, but it may require some coding
yourself.

Best wishes,

Jan-Mathijs




On 1 Dec 2009, at 00:33, Dahlia Sharon wrote:

> Hi,
>
> Since the coherence spectrum is different from the power spectrum in
> not
> being a simple average over trials, I am wondering how the point-by-
> point
> statistic should be calculated. I.e. does it make sense to take the
> difference in coherence between the two conditions and divide by the
> standard error?
>
> A shorter version of this question is: how does one test for a
> statistically
> significant coherence difference between two stimulus conditions in
> fieldtrip?
>
> Thanks,
> Dahlia.
>
> ----------------------------------
> 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://listserv.surfnet.nl/archives/fieldtrip.html
>  and http://www.ru.nl/neuroimaging/fieldtrip.
>

Dr. J.M. (Jan-Mathijs) Schoffelen
Donders Institute for Brain, Cognition and Behaviour,
Centre for Cognitive Neuroimaging,
Radboud University Nijmegen, The Netherlands
J.Schoffelen at donders.ru.nl
Telephone: 0031-24-3668063

----------------------------------
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://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.