[FieldTrip] stats on planar gradient topographies

Luisa Frei l.frei at psy.gla.ac.uk
Sat May 14 22:37:45 CEST 2011


Hi there,
I am trying to compute a simple cluster-based statistic in a between  
subjects design. I have followed the steps in the respective tutorial  
very closely.
This the code I used:

%%%%%%%%%%%%%%%%%%%%%%%%%%%
cfg = [];
cfg.method = 'montecarlo';
cfg.statistic = 'indepsamplesT';

cfg.correctm = 'cluster';
cfg.clusteralpha = 0.001;
cfg.clusterstatistic = 'maxsize';
cfg.minnbchan = 4;

cfg.tail = 0;
cfg.clustertail = 0;
cfg.alpha = 0.005;
cfg.numrandomization = 500;

design = [ones(1,13) ones(1,13)+1];

cfg.design = design;
cfg.ivar  = 1;

cfg.channel = {'MEG',sub(1).allbadchan{:}};
cfg.latency = [.15 .2];

[stat] = ft_timelockstatistics(cfg, grandavgG,grandavgE); %  
individuals kept
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

My problem is, that I'm not sure which time resolution to use. I  
would like to use a small resolution of 10 ms, but this leads to a  
fairly random pattern of significant sensors (see attached figure, on  
the right). Of course, the more time points I include, the less  
significant sensors are left over. However, another problem is, that  
the ones that are left (figure, left), are not related to big  
differences in the activation pattern at all. How is it possible that  
my significant sensors end up being so randomly distributed? Also,  
ft_timelockstatistics only comes up with one positive and one  
negative cluster., but the sign. sensors are not always connected.  
Could anyone give some helpful input here?

Thanks,
Luisa

Fyi: I have tried different thresholds and different numbers of  
neighbourhood channels, but there's not much of a change.

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