[FieldTrip] Frequency smoothing for beamforming

Stephen Whitmarsh stephen.whitmarsh at gmail.com
Thu Oct 4 15:47:21 CEST 2012

Hi Yoni,

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?

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.


On 4 October 2012 11:56, Yoni Levy <yoniilevy at gmail.com> wrote:

> 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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