[FieldTrip] Testing ITC differences using diff_itc in a first and second level analysis

[Wutz, Andreas]

Dear Nicolas,

I cannot help you on your specific question, but I can show you how I usually compare two conditions within subjects using the ITC.
You would need the CircStatToolbox (is open source), though.

After you computed your complex TF, you can get the corresponding angles in radians using: 

data.radspctrm=angle(data.fourierspctrm); 

you get the ITC for each time-frequency-sensor point by calculating the resultant vector length over the trials:

data.ITC=squeeze(circ_r(data.radspctrm, [],[],1));

Now, I simply compute the differences in ITC between the two conditions using cluster-based nonparametric statistics as implemented in FT with
cfg.parameter = 'ITC';

You may have to add one line of code, allowing ft_freqstatistics to operate with the field .ITC. 
I see no reason why a dependent samples t-test should be different for ordinary powerspectra or ITC-spectra.

I hope this helps.
Best,
Andreas

Andreas Wutz
PhD Student
CIMeC - Center for Mind/Brain Sciences
Università degli studi di Trento
________________________________________
Von: fieldtrip-bounces at science.ru.nl [fieldtrip-bounces at science.ru.nl] im Auftrag von Nicolas Farrugia [farrugia at cbs.mpg.de]
Gesendet: Mittwoch, 19. Dezember 2012 17:00
An: fieldtrip at science.ru.nl
Betreff: [FieldTrip] Testing ITC differences using diff_itc in a first and second level analysis

Dear all,

I m trying to analyse differences in Inter-Trial Coherence between conditions in a group of subjects. I figured out with this paper "Nonparametric statistical testing of coherence differences" by Maris, Schoffelen & Fries 2007, that this can be done reliably using non-parametric tests with Montecarlo permutations and it s implemented in Fieldtrip under the statfun diff_itc in the montecarlo method.
I m now using the cluster based  tests in Fieldtrip on my complex TF data using the following parameters (I extracted only the relevant ones for the discussion) :

cfg.method           = 'montecarlo';
cfg.correctm         = 'cluster';
cfg.clusteralpha     = 0.05;
cfg.clusterstatistic = 'maxsum';
cfg.minnbchan        = 2;
cfg.tail             = 0;
cfg.clustertail      = 0;
cfg.alpha            = 0.025;
cfg.numrandomization = 500;

cfg.statistic = 'diff_itc';
cfg.complex = 'absdiff';
cfg.parameter = 'fourierspctrm';
cfg.avgoverfreq = 'yes';

This also need a clustercritval : so my first question will be :

-> How to choose the clustercritval in the case of this test of ITC differences ?

I ve been trying different things but I m not sure.

Then, this constitutes my first level analysis, so for each subject I have cluster statistics, their time/spatial course and their significance.
>From here how can I go towards for a second level analysis ( similarly as in SPM) ?  Is there already building blocks for that in Fieldtrip ? Should one try to find a cluster center / time window and use the cluster statistics, and use the resulting values from all subjects in a non-parametric test ?

Thanks for your help,

Nicolas

----------------------------------------------------------------------------
-----
Dr. Nicolas Farrugia
Post-doc

Research Group "Subcortical contributions to comprehension"
Department of Neuropsychology
Max Planck Institute for Human Cognitive and Brain Sciences
Stephanstraße 1A, 04103 Leipzig, Germany
Phone : +49 (0) 176 848 90 393

http://nicolasfarrugia.fr

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