Wavelet vs. Multitaper

[Roemer van der Meij]

Hi Roni,

I haven't looked in detail at all the other settings you use, but you
apparently use a different number of cycles for the wavelets in both
methods. For the multitapers, you use 2 cycles per wavelet (which are
then convolved with the set of dpss tapers), whereas you use 1 cycle for
your Morlet wavelets (which are then convolved with a hanning taper (or
something similar)). This will make the time-window for spectral
estimation twice as long for the multitapers than for your Morlet
wavelets. You could try setting cfg.width = 2 for 'wltconvol'.

Hope it helps a bit,

Roemer



On 5/3/2010 5:08 PM, Roni Tibon wrote:
> Dear all,
>
> I've been trying to analyze the same data using both methods, but I'm
> getting very different results, depending on the method I'm using.
>
> Enclosed is a picture of the results of the analysis in 3 channels, using
> both methods. In the left column I performed a multitaper analysis for each
> channel, using the following script:
>
> cfg = [];
> cfg.output     = 'pow';
> cfg.channel    = 'A195';
> cfg.method     = 'mtmconvol';
> cfg.foi        = 1:1:50;
> cfg.t_ftimwin  = 2./cfg.foi;
> cfg.tapsmofrq  = 0.4 *cfg.foi;
> cfg.toi        = -0.2:0.05:1;
> cfg.pad        = 'maxperlen';
> f12a_a195 = ft_freqanalysis(cfg, data);
>
> In the right one, I performed wavelet analysis using:
>
> cfg = [];
> cfg.channel    = 'A195';
> cfg.method     = 'wltconvol';
> cfg.output     = 'pow';
> cfg.foi        = 1:1:50;
> cfg.width      =  1;
> cfg.toi          = -0.2:0.05:1;
> cfg.keeptrials = 'yes';
> f12b_a195=freqanalysis(cfg, data);
>
> As you can see, channel A106 looks about the same using both methods.
> However, for channels A176 and A195, the multitaper analysis revealed
> activation in frequencies ranging from ~15-25, which was not seen in the
> wavelet analysis.
>
> I used a different program (msi) to divide the trial in two (0-500ms and
> 500-1000ms) and compare activation in this frequency range between the 2
> halves, and indeed- there was more activation in the second half, as would
> be suggested by the multitaper.
>
> Can it be that when using wavelet analysis data is lost like this?
> Am I doing something wrong with the wavelet?
>
> Thanks,
> Roni
>
>
>
> ----------------------------------
> The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.
>

--
Roemer van der Meij MSc
Scientific Programmer&  Data-Analyst
Donders Institute for Brain, Cognition and Behaviour
Centre for Cognition
P.O. Box 9104
6500 HE Nijmegen
The Netherlands
Tel: +31(0)24 3612631
E-mail: r.vandermeij at donders.ru.nl

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.