Wavelet vs. Multitaper

[Roni Tibon]

Hi,
Thanks for the quick response.
I tried using the settings you suggested, which, of course, reduced the
frequency range, and revealed some activity in ~15-25hz, although it looked
minor compared to that seen in the multitaper.

I guess my questions now are these:
1. How come when I use multitaper with 1 cycle I see a range starting at 5
hz, but when I use morelet wavelet with 1 cycle, the range starts at 10hz?
2. Can I conclude that when the trial is rather short multitaper will be
more accurate, or more sensitive to changes?
3. How come the freq range in the multitaper changes depending on the
baseline settings? It started at 5hz when cfg.baseline = [-0.2 -0.05], but
started at 10hz when cfg.baseline = [-0.2 -0.1];

Thanks a lot!
Roni



On 3 May 2010 18:36, Roemer van der Meij <r.vandermeij at donders.ru.nl> wrote:

> 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.
>

----------------------------------
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.
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