clusterandanalysis: negculsters vs posclusters and onesided vs two sided tests.

[Sameer Walawalkar]

Dear Fieldtrippers,

I have been implementing clusterandanalysis (instead of freqstatistics)
according to Eric Mariss suggestion to find coherence differences between
trials. However I seem to be getting some confusing results.

  I do the following

for i = 1: numel(MEGchan)
     cfg = [];
     cfg.channelcmb = { MEGchan{i} 'MEG'};
     cfg.statistic = 'indepsamplesZcoh' ;
     cfg.onetwo = 'onesided_1<2 ;
OR    cfg.onetwo = 'onesided_2<1;
     cfg.alpha = 0.05 ;
     cfg.clusterteststat = 'maxsum' ;
     cfg.makeclusters = 'yes' ;
     cfg.minnbchan  = 2;
     cfg.nranddraws = 1000;

[clusrand] = clusterrandanalysis(cfg, CSD1 , CSD2);
clusrand_Coh_1greater2{i} = clusrand;
OR clusrand_Coh_2greater1{i} = clusrand;
OR   clusrand_Coh_twosided{i} = clusrand;
end

cfg.onetwo = 'onesided_1<2 ; as well as
cfg.onetwo = 'onesided_2<1;
find the same positive clusters (posclusters) for one of the 6 sensors
belonging to MEGchans (the other 5 return empty posclusters and
negclusters).

I would have thought the cluster which shows up as posculster in
'onesided_1<2
should show up as negcluster in 'onesided_2<1.

Secondly,
cfg.onetwo = 'twosided' ;
returns clustering effects for channels other than the one seen above  and
show only negclusters. At the same time the channel which showed
posclusters in the two one sided tests shows no custering in coherence.

Does this make sense? If yes, what is it?

Thanks a lot for your time. Your replies are always much appreciated.

Best,
sameer

p.s: Ill admit that the questions written above also partly stem from my
lack of deep understanding of clustering analysis, and in particular, I
have no idea what is meant by  negative and positive clusters. It would be
great if someone could point me to some reading material. I have tried,
but, have been unable to find any definition on negative clusters and
positive clusters, though I have some idea about how clustering works.

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