AW: [FIELDTRIP] significance of coherence differences

[Eric Maris]

Hi Jan,


> thank you for your answer. I read the paper and see that your method is
> a great tool to deal with the MCP and with situations in which one does
> not know at which frequencies and channels an effect is expected. It is
> good to know that for the group level (multiple subjects, all measured
> in the same set of conditions) conventional paired-sample statistics
> are appropriate, beacause I dont see how the random-partition procedure
> relates to such a situation.

I does not, because the random partitioning procedure only applies to single
subject studies (UOs are trials). In a within-subjects multiple subject
study the random aspect is in the random permutation of the elements in the
paired observations.


Best,

Eric


> 
> All the best,
> Jan
> 
> 
> 
> 
> -----Ursprüngliche Nachricht-----
> Von: FieldTrip discussion list im Auftrag von Eric Maris
> Gesendet: Mo 09.08.2010 18:58
> An: FIELDTRIP at NIC.SURFNET.NL
> Betreff: Re: [FIELDTRIP] significance of coherence differences
> 
> Hi Jan,
> 
> 
> 
> 
> 
> If you get confused about the literature then I am partly to blame.
> 
> 
> 
> Things are different for single subject studies (units-of-observation
> are
> trials) and multiple subject studies (units-of-observation are
> subjects).
> You describe a multiple subject study of the within-subjects type
> (every
> subjects participates in both experimental conditions). For a single
> channel
> pair and a single frequency bin, the appropriate statistic is the
> dependent
> (paired) samples t-statistic or, in a nonparametric framework, the
> Wilcoxon
> signed rank sum test. (You mention the Wilcoxon rank sum test, but this
> is
> for a study of the between-subjects type.) You apply this test
> statistic to
> the condition-specific coherences, obtained by summing and normalizing
> the
> trial(taper)-specific cross-spectra. This statistical test works fine
> (asymptotically, for the dependent samples t-statistic) if the number
> of
> trials in the two conditions are equal. The requirement of an equal
> number
> of trials follows from the fact that coherence is a biased measure,
> with the
> bias being dependent on the number of trials.
> 
> 
> 
> The situation is different for a single subject study. You may want to
> have
> a look at our paper (Maris, Schoffelen, Fries, 2007), which also deals
> with
> the multiple comparisons problem.
> 
> 
> 
> 
> 
> Best,
> 
> 
> 
> Eric
> 
> 
> 
> 
> 
> dr. Eric Maris
> Donders Institute for Brain, Cognition and Behavior
> 
> Center for Cognition and F.C. Donders Center for Cognitive Neuroimaging
> 
> Radboud University
> P.O. Box 9104
> 6500 HE Nijmegen
> The Netherlands
> T:+31 24 3612651
> Mobile: 06 39584581
> 
> F:+31 24 3616066
> E: e. <mailto:e.maris at donders.ru.nl> maris at donders.ru.nl
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> From: FieldTrip discussion list [mailto:FIELDTRIP at NIC.SURFNET.NL] On
> Behalf
> Of Jan Hirschmann
> Sent: donderdag 5 augustus 2010 15:50
> To: FIELDTRIP at NIC.SURFNET.NL
> Subject: [FIELDTRIP] significance of coherence differences
> 
> 
> 
> Dear fieldtrip experts,
> 
> 
> 
> I am getting more and more confused about the literature of
> significance
> tests of coherence differences. The one essential point I did not quite
> understand is why conventional tests do not seem to apply. Lets say I
> have
> two vectors of mean alpha band coherences, relating to two experimental
> conditions (continuous data, no trials). The vector entries stem from
> different subjects: C1=[meancoh_cond1_subj1, meancoh_cond1_subj2,.] and
> C2=[meancoh_cond2_subj1, meancoh_cond2_subj2,.]. What keeps me from
> comparing the two vectors, say, with a Wilcoxon ranksum test?
> 
> 
> 
> Thanks for any comments,
> 
> 
> 
> Jan Hirschmann
> 
> ----------------------------------
> 
> 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.
> 
> http://listserv.surfnet.nl/archives/fieldtrip.html
> 
> http://www.ru.nl/fcdonders/fieldtrip/
> 
> 
> ----------------------------------
> 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.
> 
> 
> ----------------------------------
> 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.

----------------------------------
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.