ft_freqstatistics between two groups

[David Ziegler]

Hi Fieldtrippers,

I am trying to run a comparison between TFRs from two groups of subjects
and I keep getting some errors that I can't track down answers for.  I
seem to recall something like this being discussed on this list, but I
can't for the life of me track down that thread at the moment.

I am using data from a neuromag 306 system on which I have calculated
TFRs on the gradiometer data, then ran ft_combineplanar, followed by
ft_freqdescriptives, followed by ft_freqgrandaverage for each of the two
groups with the cfg.keepindividual = 'yes' option.  I am able to plot
the grand averages just fine, so the data seem to be ok (and there are
fairly striking differences between the two).

When I run a permutation test to quantify the group differences, I get
the following error messages (although the analysis does run):

/computing statistic 500 from 500
Warning: Not all replications are used for the computation of the statistic.
 > > In statfun_indepsamplesT at 57
   In statistics_montecarlo at 298
   In fieldtrip-20100617/private/statistics_wrapper at 285
   In ft_freqstatistics at 105
Warning: Divide by zero./

When I inspect the output file, the .stat contains a single column of
NaNs.  Does anyone know what I am doing wrong here?

Here is the actual code I am using to run the statistics:

cfg = [];
cfg.channel          = {'MEG *3'}; %only combined gradiometer data
cfg.layout = 'neuromag306cmb_dz.lay';
cfg.latency     = [0.2 0.8];
cfg.avgovertime = 'yes';
cfg.avgoverchan      = 'no';
cfg.avgoverfreq      = 'yes';
cfg.parameter   = 'powspctrm';
cfg.frequency        = [14 26];
cfg.method           = 'montecarlo';
cfg.statistic        = 'indepsamplesT';
cfg.correctm         = 'no';
cfg.clusteralpha     = 0.05;
cfg.clusterstatistic = 'maxsum';
cfg.minnbchan        = 2;
cfg.tail             = 0;
cfg.clustertail      = 0;
cfg.alpha            = 0.05;
cfg.numrandomization = 500;

cfg.design           = [1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
20 21 22 23 24 25 ;   % subject number
                                     1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2
2 2 2 2 2 2 2]; % 12subjs in grp1, 13subjs in grp2
cfg.uvar = 1;
cfg.ivar = 2;

GA_low_stat = ft_freqstatistics(cfg, GA_grp1_low, GA_grp2_low)

Thanks,
David


--

David A. Ziegler

Department of Brain and Cognitive Sciences
Massachusetts Institute of Technology
43 Vassar St,46-5121
Cambridge, MA 02139

Tel: 617-258-0765

Fax: 617-253-1504

daz at mit.edu <mailto:daz at mit.edu>


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