[FieldTrip] Using Akaike Information Criterion for order selection with ft_mvaranalysis

[jan-mathijs schoffelen]

A determinant of 0 to me suggests that your data is rank deficient, which suggests the number of virtual channels is larger than the number of original observations.
Best,
Jan-Mathijs

On Sep 29, 2014, at 11:46 AM, Nitzan Uziely <linkgoron at gmail.com> wrote:

> Hi again,
> 
> Just to clarify the above, here is the code that I use to calculate the AIC:
> "source" is my inverse-solution after being partitioned into virtual channels using the AAL atlas.
> 
> % mvar analysis.
>    cfg         = [];
>    cfg.order   = order;
>    cfg.toolbox = 'bsmart';
>     mdata       = ft_mvaranalysis(cfg, source);        
>     
>     % aic calculation: www.ncbi.nlm.nih.gov/pmc/articles/PMC2585694/
>     k = size(source.label,1);
>     p = cfg.order;
>     logv = log(det(mdata.noisecov));    % this gives me inf, as det(mdata.noisecov) is 0.
> 
>     % look at source 
>     nTotal = size(source.time,2)*length(source.time{1});    
>     aic = -logv + 2*p*(k^2)/nTotal;     
>     disp(strcat(['order:' num2str(order)  ', aic:',num2str(aic)]));
> 
> Any ideas would be greatly appreciated!
> Best,
> 
> Nitzan
> 
> On Fri, Sep 26, 2014 at 9:24 PM, Nitzan Uziely <linkgoron at gmail.com> wrote:
> Hi,
> 
> My name is Nitzan Uziely, and I'm a student (under-grad) at the Hebrew University of Jerusalem.
> 
> I'm using fieldtrip to process EEG signals at our lab, and I'm trying to run a PDC analysis on my data. 
> 
> I took the data, calculated the inverse-solution and segmented it into virtual channels using the AAL atlas.
> 
> I'm trying to to calculate the correct order for the ft_mvaranalysis function.
> I've seen a previous question that went unanswered a few years ago about order selection (http://mailman.science.ru.nl/pipermail/fieldtrip/2011-June/003947.html).
> 
> Following the above question, I searched how to calculate the Akaike Information Criterion (AIC) to calculate the correct model order. According to http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2585694/ I need to calculate the determinant of the noise covariance matrix, however the determinant (I've checked orders 1 to 10) is so small that matlab just rounds it to zero. This makes me feel that either I have a problem, I've misunderstood how to calculate AIC to select the correct model, or that AIC is not the correct way to go.
> 
> So, my question is - does it seem that is something wrong with my data (which can be seen by the small determinant), am I misunderstanding the requirement of the determinant or is there a better way to select the order of the mvar model. (oh, I'm using mdcfg.toolbox = 'bsmart'; if it matters)
> 
> Thanks,
> 
> Nitzan.
> 
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Jan-Mathijs Schoffelen, MD PhD 

Donders Institute for Brain, Cognition and Behaviour, 
Centre for Cognitive Neuroimaging,
Radboud University Nijmegen, The Netherlands

Max Planck Institute for Psycholinguistics,
Nijmegen, The Netherlands

J.Schoffelen at donders.ru.nl
Telephone: +31-24-3614793

http://www.hettaligebrein.nl

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