[FieldTrip] time frequency analysis - basic question

[Belluscio, Beth (NIH/NINDS) [E]]

Thanks, Jorn, that makes sense to me.

From: "Jörn M. Horschig" [mailto:jm.horschig at donders.ru.nl]
Sent: Friday, August 05, 2011 3:30 AM
To: Email discussion list for the FieldTrip project
Subject: Re: [FieldTrip] time frequency analysis - basic question

Dear Beth,

>From all my knowledge I gathered so far, I can try to answer your question (and hope that people from this list correct me if I am wrong):
As often, the truth lies somewhere in between. The frequency smoothing is determined by the type and length of the taper you apply - this is similar to wavelet analysis. If you use multitapers, you can specify cfg.tapsmofrq, which determines the frequency band you want to smooth with. The FT freqanalysis method uses multiplication in the time domain instead of convolution in the frequency domain (which is equivalent). Any taper will always have a specific frequency spectrum itself, and thus it will not give you the value of a single frequency (though the output looks as if). So if you specify foi as [5 10 15], then the power at 5Hz will not be from 2.5 to 7.5Hz, but neither will it be at exactly and only 5Hz (btw, the exact center frequency might not be 5Hz, this depends on the (Rayleigh) frequency resolution of your signal).

I do not know any more specifics concerning this, maybe the help of ft_freqanalysis can tell you some additonal things: http://fieldtrip.fcdonders.nl/reference/ft_freqanalysis

Best,
Jörn

Btw, if this is not correct, else from the mailinglist should correct me asap ;)



On 8/4/2011 3:23 PM, Belluscio, Beth (NIH/NINDS) [E] wrote:
Hi all-
  When calling ft_freqanalysis you specify the foi limits and intervals - eg 5:5:30.  The resulting data set appears to generate the power at discreet frequencies (5, 10, 15, 20, 25, 30).  Does this actually reflect an estimate of the power over a range of frequencies (2.5-7.5, 7.5-12.5, 12.5-17.5,....) or the power at discreet frequencies with those in between not evaluated?
  Thanks for your help,
Beth.

Beth Belluscio MD-PhD
Clinical Fellow
Human Motor Control Section
NINDS, NIH
301-402-3495





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