frequency resolution of the power spectrum

Ingrid Nieuwenhuis ingrid.nieuwenhuis at DONDERS.RU.NL
Thu Jul 2 16:07:34 CEST 2009


Hi

> your 5 minutes. to do this, refer to the mail by nathan weisz "re:
> preprocessing without trigger...". he shows some code there to cut long
> trials into several short ones.

Or use statfun_general see reply on that "re: > preprocessing without
trigger..."

Best Ingrid

> -----Original Message-----
> From: FieldTrip discussion list [mailto:FIELDTRIP at NIC.SURFNET.NL] On
> Behalf Of Thomas Hartmann
> Sent: Thursday, July 02, 2009 4:00 PM
> To: FIELDTRIP at NIC.SURFNET.NL
> Subject: Re: [FIELDTRIP] frequency resolution of the power spectrum
>
> hi martin,
> the spectral resolution of a fft is determined by the length of the data
> used to calculate it. e.g. a length of 1 second would give a
> frequency-resolution of 1Hz, 2 seconds of data would result in a
> resolution of 0.5Hz and so on....
>
> what you actually missed is that you dont want to calculate one fft on
> your 5-minutes trial but rather several ffts on (overlapping) windows of
> your 5 minutes. to do this, refer to the mail by nathan weisz "re:
> preprocessing without trigger...". he shows some code there to cut long
> trials into several short ones. calculate the freqanalysis on these and
> you will get reasonable results.
>
> greez,
> thomas
>
> Martin Bleichner schrieb:
> > Hi there,
> >
> > this might be a stupid question. I probably miss something obvious.
> >
> > I want to compute the power spectrum over a single trial (around 5
> > minutes).
> >
> > using this piece of code
> > cfg        = [];
> > cfg.method = 'mtmfft';
> > cfg.output = 'pow';
> > cfg.foilim = [1 100];
> > cfg.taper  = 'hanning';
> > freq       = freqanalysis(cfg, data);
> >
> > as a result I get
> >  freq: [1x62534 double]
> > powspctrm: [96x62534 double]
> >
> > that is a frequency resolution of 0.0032Hz!? What determines the
> > frequency resolution here? What can I do to make it less fine grained?
> >
> > Thanks
> > Martin
> >
> >
> > ----------------------------------
> >
> > 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.
> >
> > http://listserv.surfnet.nl/archives/fieldtrip.html
> >
> > http://www.ru.nl/fcdonders/fieldtrip/
> >
>
>
> --
> Dipl. Psych. Thomas Hartmann
>
> OBOB-Lab
> University of Konstanz
> Department of Psychology
> P.O. Box D25
> 78457 Konstanz
> Germany
>
> Tel.: +49 (0)7531 88 4612
> Fax: +49 (0)7531-88 4601
> Email: thomas.hartmann at uni-konstanz.de
> Homepage: http://www.uni-konstanz.de/obob
>
> "I am a brain, Watson. The rest of me is a mere appendix. " (Arthur Conan
> Doyle)
>
> ----------------------------------
> 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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