[FieldTrip] Frequency smoothing for beamforming

[Stephen Whitmarsh]

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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