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

Gregor Volberg Gregor.Volberg at psychologie.uni-regensburg.de
Thu Nov 3 18:20:56 CET 2011


Hi Michael, 

thanks a lot for your response. Yes, I kept everything the same, the only difference beeing the Matlab path pointing to the old vs new FT version. I will keep on searching for a possible flaw in my code and will tell so if I found one. 
Thanks  again, best regards, 
Gregor 



-- 
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


>>> "Michael Wibral" <michael.wibral at web.de> 11/3/2011 4:57 PM >>>
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 )

  
 
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