[FieldTrip] MOrler wavelet nonconstant Q

[Till Schneider]

Dear Fanny,

we used wavelets with linearly increasing Q. Q defines the temporal and 
spectral resolution of the wavelet.
In fieldtrip you have to change the parameter cfg.width, in order to 
adjust Q for the wavelet analysis using ft_freqanalysis.
You could use a linear increasing value for cfg.width dependent on the 
frequency of interest, e.g. cfg.width = linspace(8.5, 13, 
length(30:2:100)).
Together with a frequency range from 30 to 100 Hz (cfg.foi = 30:2:100), 
this would yield wavelets with increasing Q across frequencies.

cfg = [];
cfg.channel    = 'EEG';	
cfg.method     = 'wavelet';
cfg.width      = linspace(8.5, 13, length(30:2:100));
cfg.output     = 'pow';	
cfg.foi        = 30:2:100;	
cfg.toi        = -0.5:0.05:1.5;		
TFRwavelet = ft_freqanalysis(cfg, data);

Best,
Till

-- 

Till Schneider, PhD

Cognitive and Clinical Neurophysiology Group
Dept. of Neurophysiology and Pathophysiology
University Medical Center Hamburg-Eppendorf
Martinistr. 52
20246 Hamburg
Germany

phone +49-40-7410-53188
fax +49-40-7410-57126
www.uke.de/neurophysiologie





Am 03.12.12 13:55, schrieb "Jörn M. Horschig":
> Dear Fanny,
>
> I have no experience in using this nonconstant Q wavelet, but I hope I 
> can shed some light on the terminology.
>
>> "
>> /Time--frequency analysis was performed for
>> each channel by convolving the data with a complex Morlet
>> wavelet w(t, f0) having a Gaussian shape in time (?t) and in
>> frequency (?f) around the center frequency ( f0). Nonconstant
>> wavelets with Q increasing from f0/?f =8.5 to 13 for frequencies
>> from 30 to 100 Hz (step size 2 Hz) were used.
>> /
>> "
>>
>>
>> -  I don't understand what is /f/_0 and  ?/_f /
>
> If you compute the frequency spectrum of any signal, you will see some 
> peaks in the frequency spectrum. If you compute the frequency spectrum 
> of a wavelet, the maximal peak is conventionally called center 
> frequency. In Matlab, you can obtain the center frequency by the 
> function 'centfrq'. Note that the center frequency also depends on the 
> scale of your wavelet.
>
> Any peak can be wide or narrow, and just like in statistics you can 
> describe this with the standard deviation. I am not in particular 
> familiar with/?f /and the method you are describing, but I would guess 
> that it is really just the standard deviation, given that /? /is 
> conventionally used to denote the standard deviation.
>
> Judging from your questions, without offending, that you are probably 
> new to the field of wavelets, you should probably start off with 
> reading more general literature on wavelets. Maybe start off with 
> Wikipedia:
> http://en.wikipedia.org/wiki/Wavelet
> http://en.wikipedia.org/wiki/Morlet_wavelet
>
> and then go to some more sophisticated literature ;)
>
> Best,
> Jörn
>
>
> -- 
> Jörn M. Horschig
> PhD Student
> Donders Institute for Brain, Cognition and Behaviour
> Centre for Cognitive Neuroimaging
> Radboud University Nijmegen
> Neuronal Oscillations Group
> FieldTrip Development Team
>
> P.O. Box 9101
> NL-6500 HB Nijmegen
> The Netherlands
>
> Contact:
> E-Mail:jm.horschig at donders.ru.nl
> Tel:    +31-(0)24-36-68493
> Web:http://www.ru.nl/donders
>
> Visiting address:
> Trigon, room 2.30
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> NL-6525 EN Nijmegen
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>
>
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