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

[Gregor Volberg]

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


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