Freqstatistics actvsblT vs. depsamplesT

Ingmar Schneider Ingmar.Schneider at BIO.UNI-GIESSEN.DE
Tue Jun 23 12:35:45 CEST 2009


Dear fieldtrip community,

I am currently working on an activation vs. baseline comparison script  
(attached below) using the 'freqstatistics' function provided by  
fieldtrip. According to the instructions given in the fieldtrip  
tutorial, I used the 'actvsblT'-statfun for the comparison of 2s long  
data segments over a frequency range from 6Hz to 100Hz. However, this  
turned out to be almost impossible due to huge RAM requirements and  
frequently led to an 'Out of memory-error' even on a machine with 32GB  
of RAM.
In order to make it work, I had to narrow the frequency range and  
number of randomizations. When I used the 'depsamplesT'-statfun for  
the comparison instead of 'actvsblT', a comparison across the whole  
frequency-range was possible requiring much less RAM (~2GB). The  
resulting statistics, however, slightly varied from those obtained  
using the 'actvsblT'-statfun (Topoplots attached); the localisation  
seems to be the same, but the t-values are a little lower.

As far as I could find out using the documentation both statfuns  
calculate dependent samples T-statistics. Is there an essential  
difference between them causing the differences in the statistics?

With best regards,
Ingmar


% Preparation of the statistical design
nsubjects   = size(TFact, 2);
statdesign  = zeros(2,2*nsubjects);
statdesign(1,1:nsubjects)               = 1;
statdesign(1,nsubjects+1:2*nsubjects)   = 2;
statdesign(2,1:nsubjects)               = [1:nsubjects];
statdesign(2,nsubjects+1:2*nsubjects)   = [1:nsubjects];

% Realigning time
TFblavg.time   = TFactavg.time;

% Freqstatistics
cfg = [];
cfg.frequency   = [9 11];
cfg.channel     = {'MEG', '-MLP12', '-MRC14', '-MLT41', '-MRC25',  
'-MRP56', '-MRT21', '-MLO21'};
cfg.latency     = [0.5 2.5];
cfg.parameter   = 'powspctrm';
cfg.method      = 'montecarlo';
cfg.statistic   = 'depsamplesT' / 'actvsblT' % respectively
cfg.correctm    = 'fdr';
cfg.alpha       = 0.05;
cfg.numrandomization    = 1500;
cfg.grad        = grad;
cfg.design      = statdesign;
cfg.uvar        = 2;
cfg.ivar        = 1;
freqstatACTvsBL = freqstatistics(cfg, TFactavg, TFblavg);


-- 
Ingmar Schneider

Max-Planck-Institut f├╝r Hirnforschung
Deutschordenstra├če 46
D-60528 Frankfurt/Main

Tel.: 069/6301-83221
Fax: 069/96769-327
Mail1: schneider at mpih-frankfurt.mpg.de
Mail2: ingmar.schneider at bio.uni-giessen.de

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