[FieldTrip] Frequency smoothing for beamforming

[Roemer van der Meij]

Hi Yoni,

Just to chime in quickly, please keep in mind that a tapsmofrq of 8 Hz
actually indicates a smoothing window of 16Hz. I.e. cfg.tapsmofrq specifies
the half-width of your smoothing window. So a tapsmofrq of 8 would mean
that, for 30Hz, you are looking at signal coming from 22Hz to 38Hz. Also,
the efficacy of the smoothing is largely dependent on the number of tapers
(the more tapers, the closer your smoothing is to a 'box').

All the best,
Roemer



On Wed, Oct 3, 2012 at 3:42 PM, Stephen Whitmarsh <
stephen.whitmarsh at gmail.com> wrote:

> Hi Yoni!
>
> The extend of the smoothing, I would say, is under normal circumstances
> simply what you
> request as a smoothing paramater (given the dpss characteristics), so I
> don't understand
> that formulation exactly.
>
> If different smoothings give drastically different result you might be
> sampling
> frequencies that behave differently from your frequency of interest. In
> your case, e.g.
> perhaps you are adding alpha in your estimate that might behave
> differently in your
> paradigm?
>
> I would therefor try to first figure out if your effect is, in fact,
> frequency specific
> and try to not to smooth more than necessary to capture that effect. So
> starting with no
> (extra) smoothing and looking at the TFR for instance. A simple FFT would
> give you a
> frequency smoothing of +/- 1/datalength already (e.g. half a second would
> be +/- 2 Hz).
> Simply averaging over frequencies (estimated with a Hanning taper) instead
> of using the
> slepian tapers might be a better option.
>
> Then again, you are limited in frequency specificity by the length of the
> data on which
> you calculate them. If that is too short you might have suboptimal and
> unexpected
> effects. In the case of slepian filters make sure you have at least a
> minimum of 3 tapers
> (which is shown in the output of freqanalysis).
>
> There is a lot more to say about tapers, smoothing etc, but I hope this
> helps.
>
> All the best,
> Stephen
>
> On 3 October 2012 15:14, Yoni Levy <yoniilevy at gmail.com> wrote:
>
>> Dear Fieldtrippers,
>>
>> I am trying to locate the source of an oscillatory effect at the
>> frequency of 30Hz in a time window of interest.
>> Before running the ft_sourceanalysis function, I run a ft_freqanalysis
>> with a frequency smoothing of 8 (cfg.tapsmofrq =8).
>> My question is whether there is any rule of thumb by which I could
>> reliably determine the extent of the smoothing?
>> I found out that even small changes in the 'tapsmofrq' value,
>> significantly alter the spatial localization of the resulting sources.
>> For instance, a tapsmofreq value of 8 would point to an effect in the
>> frontal lobe, whereas a value of 10 would point to an effect in the
>> parietal lobe.
>>
>> Any advice would be appreciated.
>>
>> Yoni
>>
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>
>
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-- 
Roemer van der Meij M.Sc.
PhD student
Donders Institute for Brain, Cognition and Behaviour
Centre for Cognition
P.O. Box 9104
6500 HE Nijmegen
The Netherlands
Tel: +31(0)24 3655932
E-mail: r.vandermeij at donders.ru.nl
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