[FieldTrip] Frequency smoothing for beamforming (Roemer van der Meij)

[Yoni Levy]

Hi Roemer,

Sure, thanks.
I am just noting that even tiny differences in both the values of
'tapsmofrq' that i feed in freqanalysis, and in 'cfg.minlength' that i feed
in the redefinetrial function (just before running the freqanalysis) --
significantly alter the localization of sources that beamformer is (or is
not) outputting.

Best,
Yoni


On Thu, Oct 4, 2012 at 12:00 PM, <fieldtrip-request at science.ru.nl> wrote:

>
>
> 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
> >
>
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