Clusterplot not highlighting clusters & average layout file

Manish Saggar manish.saggar at GMAIL.COM
Wed Sep 9 18:56:36 CEST 2009

Dear All,

I am a novice user of FieldTrip and I love the tools provided by you
guys. Great work.

I have a question, regarding the analysis I recently did. Any help is
appreciated. I am confused why the cluster is not mapped on the areas
that visually show clear reduction.

I am comparing two conditions (with-in subjects) using depSamplesT
statistic. Both of the conditions are 1 min of EEG data and I
calculate powerspectrum over those 1 min windows using segment of 1
second windows with 50% overlap. Condition are named as pre and post.

I have attached topoplotER of power spectrum for each condition in
alpha band (8-12Hz). Clearly in post there is reduced power in left
(mainly) and right hemispheres at the back of the head as compared to
pre. However in the clusterplot (raweffect) the cluster is mainly
situated in midline frontal and parietal areas. Actually it is quite a
large cluster, covering almost all channels. The thing that worries me
is that it just didn't cover the the channels which clearly shows
difference by visual inspection between two conditions.
Also shouldn't the cluster be located only (or mainly) where the
difference can be seen in pre and post topoplots.

For convenience I have plotted all plots with same zlim.

Here is the code that I used for running stats:
       cfg = [];          = {'all'};
       cfg.latency          = 'all';
       cfg.frequency        = foi;
       cfg.method           = 'montecarlo';
       cfg.statistic        = depsamplesT;
       cfg.correctm         = 'cluster';
       cfg.clusteralpha     = 0.05;
       cfg.clusterstatistic = 'maxsum';
       cfg.minnbchan        = 2;   %works best.
       cfg.tail             = 0;
       cfg.clustertail      = 0;
       cfg.alpha            = 0.025;
       cfg.numrandomization = 1000;

       subj = 28;

       design = zeros(2,2*subj);
       for i = 1:1:subj
          design(1,i) = i;
          design(1,subj+i) = i;
       design(2,1:subj) = 1;
       design(2,subj+1:2*subj) = 2; = design;
       cfg.uvar = 1;
       cfg.ivar = 2;
       cfg.layout = 'ng.sfp';
       cfg.neighbourdist = 0.1; % this gives the neighbor count on
average to 3.2
       cfg.avgoverfreq = 'yes';
       stat{win} = freqstatistics(cfg, post, pre);


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