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<div class="moz-cite-prefix">Hey Yoni,<br>
<br>
Stephen is right, and just to make this really clear, a Hanning
taper will always give you a smoothing of your Raleigh frequency
(which in your case is 3.33Hz). Any taper can only (effectively)
smooth in terms of your frequency resolution or Raleigh frequency,
thus a Hann taper gives you the minimal smoothing (apart from a
boxcar). Then, the problem with different trial becomes more
apparent, because since the frequency resolution changes, also the
smoothing of the Hanning taper changes accordingly. I also think
that making the trials having equal length is the best approach.
Having unequal trial lengths also constitutes a problem for
multitapering, cause you will end up with different tapers and
different number of tapers per trial. And also your frequency
smoothing should be a multiple of the Raleigh frequency. You can
ask for other smoothing, e.g. 8Hz with 3.33Hz resolution, but
effectively you will see the smoothing at 6.66 or 9.99Hz
(depending on where you define the end of smoothing) - it's just
because you sample in 3.33Hz steps. Here you can maybe also see,
that having different trial lengths might constitute a problem,
because you will effectively get different smoothing per trial,
depending on your Raleigh frequency. The computation of the tapers
was however correct, so with 8Hz smoothing and a 0.3s time window
you get 3 tapers ;) Btw, I once played around with it and realized
that the 3 tapers you obtain are not always the same for different
parameters, e.g. for 8Hz and 0.25s window you will also get
8*0.25*2-1 = 3 tapers, but they will be different from the 3
tapers you get with a 0.3s time window. So even that can cause a
problem.<br>
<br>
Btw, I never heard that different frequency smoothing ends up in
different part of the brain when beaming. The only reason I can
see is what Stephen already pointed out, that other frequency
bands with different functional characteristics smear into your
power spectrum. <br>
<br>
Best,<br>
Jörn<br>
<br>
<br>
<br>
<br>
On 10/4/2012 3:47 PM, Stephen Whitmarsh wrote:<br>
</div>
<blockquote
cite="mid:CAFrxm=yTeqtaGAZ6Ws-AZKPABCCGCz8GsFr-1TSTx_8tTmBTPA@mail.gmail.com"
type="cite">Hi Yoni,<br>
<br>
Indeed, a simple hanning taper will already give you a frequency
smoothing of +/- 3Hz. Adding tapers can only increase this, and I
don't see why you would beamform 22 to 38 Hz if you are interested
between in 29-31 Hz. Couldn't you just do cfg.foi = 30, with
cfg.taper = 'hanning', giving you a measure of power between of
about 27 and 33? <br>
<br>
You're right that having different trial lenghts will indeed give
you a different frequency resolution per trial. If this is a
problem is hard to say from here. cfg.minlength = 'maxperlen in
ft_redefinetrial would indeed make sure they are all of the same
length (i.e. the maximal length) - but if that is different
between subjects/conditions that might not be enough.<br>
<br>
Best,<br>
Stephen<br>
<br>
<br>
<br>
<div class="gmail_quote">On 4 October 2012 11:56, Yoni Levy <span
dir="ltr"><<a moz-do-not-send="true"
href="mailto:yoniilevy@gmail.com" target="_blank">yoniilevy@gmail.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="ltr">Hi Stephen! <br>
Thanks for your reply. <br>
<br>
My FOI is 29-31Hz; Since my time window is of 300ms, then my
freq smoothing should now be of +/-3.33Hz. If I use a
hanning taper, the parameters that i use for the freqanal
(for further on doing beamformer-statistics) are:<br>
cfg.method ='mtmfft';<br>
cfg.output ='fourier';<br>
cfg.keeptrials = 'yes';<br>
cfg.keeptapers = 'yes';<br>
cfg.taper = 'hanning';<br>
cfg.foilim = [29 31];<br>
However, if I get it right, multitapering should also be an
option as 30Hz is not a relatively very low frequency. In
that case, i remove the hanning and instead include a
cfg.tapsmofrq =8, so that the number of tapers results in
8*0.3*2-1= 3 (I think?). Is it so?<br>
<br>
Also, about the time window which is theoretically 300ms,
but i think this depends on the length of every trial; for
instance, before freqanal, when i redefine the trial, i
input cfg.minlength = 'maxperlen'. So if i alter that, the
freq smoothing should be different as well, correct? Ye,
anyway, I wonder how to optimize all those parameters for my
source localization statistics. <br>
<br>
Thanks in advance,<br>
<br>
Yoni
<div>
<div class="h5"><br>
<br>
<div class="gmail_quote">On Wed, Oct 3, 2012 at 3:55 PM,
<span dir="ltr"><<a moz-do-not-send="true"
href="mailto:fieldtrip-request@science.ru.nl"
target="_blank">fieldtrip-request@science.ru.nl</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex"><br>
Hi Yoni!<br>
<br>
The extend of the smoothing, I would say, is under
normal circumstances<br>
simply what you<br>
request as a smoothing paramater (given the dpss
characteristics), so I<br>
don't understand<br>
that formulation exactly.<br>
<br>
If different smoothings give drastically different
result you might be<br>
sampling<br>
frequencies that behave differently from your
frequency of interest. In<br>
your case, e.g.<br>
perhaps you are adding alpha in your estimate that
might behave differently<br>
in your<br>
paradigm?<br>
<br>
I would therefor try to first figure out if your
effect is, in fact,<br>
frequency specific<br>
and try to not to smooth more than necessary to
capture that effect. So<br>
starting with no<br>
(extra) smoothing and looking at the TFR for
instance. A simple FFT would<br>
give you a<br>
frequency smoothing of +/- 1/datalength already
(e.g. half a second would<br>
be +/- 2 Hz).<br>
Simply averaging over frequencies (estimated with a
Hanning taper) instead<br>
of using the<br>
slepian tapers might be a better option.<br>
<br>
Then again, you are limited in frequency specificity
by the length of the<br>
data on which<br>
you calculate them. If that is too short you might
have suboptimal and<br>
unexpected<br>
effects. In the case of slepian filters make sure
you have at least a<br>
minimum of 3 tapers<br>
(which is shown in the output of freqanalysis).<br>
<br>
There is a lot more to say about tapers, smoothing
etc, but I hope this<br>
helps.<br>
<br>
All the best,<br>
Stephen<br>
<br>
On 3 October 2012 15:14, Yoni Levy <<a
moz-do-not-send="true"
href="mailto:yoniilevy@gmail.com" target="_blank">yoniilevy@gmail.com</a>>
wrote:<br>
<br>
> Dear Fieldtrippers,<br>
><br>
> I am trying to locate the source of an
oscillatory effect at the frequency<br>
> of 30Hz in a time window of interest.<br>
> Before running the ft_sourceanalysis function,
I run a ft_freqanalysis<br>
> with a frequency smoothing of 8 (cfg.tapsmofrq
=8).<br>
> My question is whether there is any rule of
thumb by which I could<br>
> reliably determine the extent of the smoothing?<br>
> I found out that even small changes in the
'tapsmofrq' value,<br>
> significantly alter the spatial localization of
the resulting sources.<br>
> For instance, a tapsmofreq value of 8 would
point to an effect in the<br>
> frontal lobe, whereas a value of 10 would point
to an effect in the<br>
> parietal lobe.<br>
><br>
> Any advice would be appreciated.<br>
><br>
> Yoni<br>
><br>
</blockquote>
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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
FieldTrip Development Team
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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The Netherlands</pre>
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