[FieldTrip] Wrong clustering of electrodes in newer FT versions?

["Jörn M. Horschig"]

Hi Gregor,

did you check your neighbourselection? Maybe something is going wrong 
there, given that a lot in this code has changed since 2010. Could you 
check that using ft_neighbourplot? This would be the most logical 
explanation why the results differ. It does not really make sense that 
these three sensors form one cluster, because they should not be 
neighbours of each other. So i would suppose that something is going 
wrong there.

Best,
Jörn


On 11/3/2011 4:57 PM, Michael Wibral wrote:
> Hi Gregor,
>
> in principle the first level statistics that determine cluster 
> membership (and thereby cluster shapes etc.)  can also vary from one 
> set of permutations to the next, esp. on the borders of a cluster 
> (this is why it is typically said that the localization of a cluster 
> is not as trustworthy as its existence). Other resaons for differences 
> are differing number of permutations - did you set everything the same?
>
> This said there might well be another problem - I hope the cluster 
> people can have a look into it.
>
> Best ,
> Michael
>
>
>     ------------------------------------------------------------------------
>     *Von:* "Gregor Volberg" <Gregor.Volberg at psychologie.uni-regensburg.de>
>     *Gesendet:* Nov 2, 2011 6:34:43 PM
>     *An:* fieldtrip at donders.ru.nl
>     *Betreff:* [FieldTrip] Wrong clustering of electrodes in newer FT
>     versions?
>
>     Dear fieldtrip community,
>
>
>     I just encountered an unexpected behavior of FT that I would like
>     to share with the list. It is that the cluster permutation test
>     formed incorrect clusters from a given set of significant
>     electrodes in newer FT versions (ft-20111012; I also tried with
>     ft-20111028).
>
>
>     For example, when using an old ft version (ft-20100810)  and
>     freqstatistics I get a negative cluster with 17 contiguous
>     electrodes. With the newer ft version, using the same data and the
>     same cfg, three significant  electrodes that are spatially
>     contiguous to the other significant electrodes are not included in
>     the cluster. Also, three electrodes that are spatially seperated
>     and are actually part of the first cluster are grouped into a
>     separate cluster.  I attached a picture with the respective
>     clusterplots where differences are marked with red arrows.
>
>     I am aware that the permutation p-value for a given cluster can be
>     different in two separate analyses, but the grouping of
>     significant electrodes into clusters should always be the same, am
>     i right? I double-checked the cfg.neighbours, after conversion to
>     new 'struct' style, and they were identical in the "old ft" and
>     "new ft"  analysis. Also, the stat.stat field and the
>     stat.cfg.clustercritval were the same for both analyses. Thus, the
>     same neighbourhood relations and the same significant electrodes
>     should be used for grouping. Yet I get different results...
>
>
>     I am almost sure that this is trivial, but I could not yet
>     replicate my old results with new FT and this makes me nervous.
>     Has someone made a similar experience? Or do newer versions of ft
>     use some further information besides cfg.neighbours for
>     clustering, somewhere in the *cfg.previuous perhaps, that I am not
>     aware of? Any comments are highly welcome.
>
>
>     Best regards,
>
>     Gregor
>
>
>     cfg = [];
>
>     cfg.neighbours    = neighbours;
>
>     cfg.minnbchan=1;
>
>     cfg.avgoverfreq   = 'yes';
>
>     cfg.avgovertime   = 'yes';
>
>     cfg.method        = 'montecarlo';
>
>     cfg.correctm       = 'cluster';
>
>     cfg.statistic     = 'depsamplesT';
>
>     cfg.tail          = 0;
>
>     cfg.alpha         = 0.05;
>
>     cfg.clusteralpha = 0.05;
>
>     cfg.numrandomization = 1000;
>
>     cfg.design = [[1:14 1:14]; [zeros(1,14)+1 zeros(1,14)+2]];
>
>     cfg.uvar          = 1;
>
>     cfg.ivar          = 2;
>
>     cfg.latency       = [2.24 2.53];
>
>     cfg.frequency     = [17 21];%
>
>     stat = ft_freqstatistics(cfg, DC, TC);
>
>
>     --
>     Dr. rer. nat. Gregor Volberg
>     <gregor.volberg at psychologie.uni-regensburg.de> (
>     mailto:gregor.volberg at psychologie.uni-regensburg.de )
>     University of Regensburg
>     Institute for Experimental Psychology
>     93040 Regensburg, Germany
>     Tel: +49 941 943 3862
>     Fax: +49 941 943 3233
>     http://www.psychologie.uni-regensburg.de/Greenlee/team/volberg/volberg.html
>     <http://www.psychologie.uni-regensburg.de/Greenlee/team/volberg/volberg.html%0A>
>
>
>
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-- 
Jörn M. Horschig
PhD Student
Donders Institute for Brain, Cognition and Behaviour
Centre for Cognitive Neuroimaging
Radboud University Nijmegen
Neuronal Oscillations Group

P.O. Box 9101
NL-6500 HB Nijmegen
The Netherlands

Contact:
E-Mail: jm.horschig at donders.ru.nl
Tel:    +31-(0)24-36-68493
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Visiting address:
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