[FieldTrip] Frequency smoothing for beamforming

[Yoni Levy]

Hi Stephen!
Thanks for your reply.

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:
        cfg.method ='mtmfft';
        cfg.output ='fourier';
        cfg.keeptrials = 'yes';
        cfg.keeptapers = 'yes';
        cfg.taper = 'hanning';
        cfg.foilim = [29 31];
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?

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.

Thanks in advance,

Yoni

On Wed, Oct 3, 2012 at 3:55 PM, <fieldtrip-request at science.ru.nl> 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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