AW: [FIELDTRIP] significance of coherence differences

Jan Hirschmann Jan.Hirschmann at MED.UNI-DUESSELDORF.DE
Tue Aug 10 10:47:52 CEST 2010


Dear Eric,

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.

All the best,
Jan




-----Urspr√ľngliche 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

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FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and
EEG analysis.

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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.



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