[FieldTrip] problem using dpss

[jan-mathijs schoffelen]

Dear Frederic,

You may want to have a look at:


http://fieldtrip.fcdonders.nl/faq/why_does_my_tfr_look_strange


Cheers,

JM


On Mar 8, 2011, at 5:44 PM, Frederic Roux wrote:

>
> Dear fieldtrip users,
>
> I have encountered a problem when using dpss to calculate spectras  
> for my data
> and I was wondering if anyone could help me to determine the origin  
> of this problem.
>
> The figure in the attachement shows the single trial MEG  
> spectrograms for channel 'MLC11'.
> I have used fieldtrip, custom code and an equivalent function from  
> the chronux toolbox to compute
> these spectrograms.
>
> The upper row of the figure shows the PSD and the spectrograms  
> calculated with the different functions
> for trial n°1.
>
> The lower row of the figure shows the PSD and the spectrograms  
> calculated with the different functions
> for trial n°2.
>
> As you can see, for trial n°2 the results show strange artifacts.
>
> The parameters I used across all computations are:
>
> window length = 0.5 Sec
> time steps = 0.025 Sec
> spectral smoothing = 10 Hz
> number of tapers = 2*(.5*10)-1 = 9
>
> Here is the fieldtrip code I used:
>
> cfg = [];
> cfg.method = 'mtmconvol';
> cfg.output = 'pow';
> cfg.taper = 'dpss';
> cfg.foi = [20:100];
> cfg.t_ftimwin = .5*ones(1,length(cfg.foi));
> cfg.tapsmofrq = 10*ones(1,length(cfg.foi));
> cfg.toi = data.time{1}(1):0.025:data.time{1}(end);
>
> SPECGRAM = freqanalysis(cfg,data);
>
> Has anyone ever encountered similar artifacts?
> Does anyone know the origin of such artifacts?
>
> Any help would as always be greatly appreciated.
>
> Frederic
> <codecheck2.jpg>_______________________________________________
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Dr. J.M. (Jan-Mathijs) Schoffelen
Donders Institute for Brain, Cognition and Behaviour,
Centre for Cognitive Neuroimaging,
Radboud University Nijmegen, The Netherlands
J.Schoffelen at donders.ru.nl
Telephone: 0031-24-3614793

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