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Hi Ion,<br>
<br>
A frequency is by definition at least a peak and a trough(up flank
and down flank), making it one cycle (think of sinusoid here). Thus,
you could say that one needs at least one cycle of a certain
frequency such that it is eligible to talk about a 'frequency'. Just
in case you don't know it already, you might want to look things up
as Rayleigh frequency, Nyquist frequency, and I would suggest that
you read (or start reading) the digitial signal processing book:
<a class="moz-txt-link-freetext" href="http://www.dspguide.com/">http://www.dspguide.com/</a><br>
<br>
Note that a true oscillation is usually considered to last at least
three cycles (of course, opinions differ her - I go with four). The
length of the window of interest reflects the number of cycles that
can be estimated. For source reconstruction, make sure cfg.toilim is
as long as the number of cycles you want to estimate, e.g. when
using cfg.foi = 10, then cfg.toilim(2)-cfg.toilim(1) should be at
least 400ms when you want an estimate of 4 cycles.<br>
<br>
Hope it helps!<br>
Best,<br>
Jörn<br>
<br>
<br>
<br>
On 3/27/2012 12:46 PM, Ion Lavado wrote:
<blockquote
cite="mid:CAMLwyK7xrbbd6_2=0MiJyXQ2OGjESnLn6TnK1R7Tqzanywk9gA@mail.gmail.com"
type="cite">Thank you Stephan, just one thing, why i need a time
window big enough? so i'm just studying the frequency.<br>
<br>
Best,<br>
<br>
Mikel<br>
<br>
<div class="gmail_quote">2012/3/27 Stephan Moratti <span
dir="ltr"><<a moz-do-not-send="true"
href="mailto:smoratti@psi.ucm.es">smoratti@psi.ucm.es</a>></span><br>
<blockquote class="gmail_quote" style="margin:0pt 0pt 0pt
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div style="word-wrap:break-word">Dear Mikel,
<div><br>
</div>
<div>I understand that you should do a mtmfft with a center
frequency of interest (in your case cfg.foil = [11 11])
and then you do a frequency smoothing of 3 Hz
(cfg.tapsmofrq = 3). Of course you need a time window big
enough to allow such a smoothing. The CSD should represent
a frequency window of 8 to 14 Hz then. Then you put the
result of the mtmfft into the dics beamformer.</div>
<div><br>
</div>
<div>Best,</div>
<div><br>
</div>
<div>Stephan</div>
<div> <br>
<div>
<div>El 27/03/2012, a las 11:19, Ion Lavado escribió:</div>
<br>
<blockquote type="cite">
<div>
<div class="h5"><span>Hello, i'm having problems
trying the beamformer analysis with a frequency
band of interest. I would like to have for
example an interval like [8 14] Hz. My concept
problem is about the cfg.frequency and
cfg.tapsmofrq values. (it must be a escalar,
but i want a frequency band). </span>
<div>
<br>
</div>
<div>%FREQ ANALYSIS</div>
<div>
<div>cfg = [];</div>
<div>cfg.method = 'mtmfft';</div>
<div>cfg.output = 'powandcsd';</div>
<div>cfg.tapsmofrq = 5;</div>
<div>cfg.foilim = [8 14];</div>
<div>freq_words_alpha = ft_freqanalysis(cfg,
words);</div>
</div>
<div><br>
</div>
<div><br>
</div>
<div>%SOURCE ANALYSIS</div>
<div>
<div>cfg = []; </div>
<div>cfg.frequency = 11; </div>
<div>
cfg.lambda=0;</div>
<div>cfg.method = 'dics';</div>
<div>cfg.coordsys = 'neuromag';</div>
<div>cfg.projectnoise = 'yes';</div>
<div>cfg.grad =
freq_words_alpha.grad;</div>
<div>
cfg.channel = {'MEG'}; </div>
<div>cfg.grid = grid; </div>
<div>cfg.reducerank = 2;</div>
<div>cfg.vol = vol02;</div>
<div>vol02=ft_convert_units(vol02,'cm');</div>
<div>source_pre11 = ft_sourceanalysis(cfg,
freq_words_alpha );</div>
</div>
<div><br>
</div>
<div><br>
</div>
<div>I use a cfg.frequency of 11 but i don't have
really clear what i'm doing as a get the same
result using cfg.frequency=11 but a freqanalysis
in [10 12]. I would be very grateful if someone
could solve my doubt.</div>
<div><br>
</div>
<div>Best wishes,</div>
<div><br>
</div>
<div>Mikel</div>
</div>
</div>
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<br>
________________________________________________________</div>
<div>Stephan Moratti, PhD<br>
<br>
</div>
<div>see also: <a moz-do-not-send="true"
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target="_blank">http://web.me.com/smoratti/</a><br>
<br>
</div>
<div>
Universidad Complutense de Madrid</div>
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<div>Campus de Somosaguas</div>
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<div>Spain</div>
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<div>and</div>
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</div>
<div>Center for Biomedical Technology</div>
<div>Laboratory for Cognitive and
Computational Neuroscience</div>
<div>Parque Científico y Tecnológico de
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<div>28223 Pozuelo de Alarcón (Madrid)</div>
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<br>
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<pre class="moz-signature" cols="72">--
Jörn M. Horschig
PhD Student
Donders Institute for Brain, Cognition and Behaviour
Centre for Cognitive Neuroimaging
Radboud University Nijmegen
Neuronal Oscillations Group
P.O. Box 9101
NL-6500 HB Nijmegen
The Netherlands
Contact:
E-Mail: <a class="moz-txt-link-abbreviated" href="mailto:jm.horschig@donders.ru.nl">jm.horschig@donders.ru.nl</a>
Tel: +31-(0)24-36-68493
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Visiting address:
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NL-6525 EN Nijmegen
The Netherlands</pre>
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