[FieldTrip] problem using dpss

[Frederic Roux]

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
 		 	   		  
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20110308/ff43aa34/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: codecheck2.jpg
Type: image/jpeg
Size: 346932 bytes
Desc: not available
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20110308/ff43aa34/attachment-0001.jpg>