# [FieldTrip] Beamforming analysis in frequency band, concept problem

"Jörn M. Horschig" jm.horschig at donders.ru.nl
Tue Mar 27 13:27:27 CEST 2012

```Hi Ion,

A frequency is by definition at least a peak and a trough(up flank and
down flank), making it one cycle (think of sinusoid here). Thus, you
could say that one needs at least one cycle of a certain frequency such
that it is eligible to talk about a 'frequency'. Just in case you don't
know it already, you might want to look things up as Rayleigh frequency,
Nyquist frequency, and I would suggest that you read (or start reading)
the digitial signal processing book: http://www.dspguide.com/

Note that a true oscillation is usually considered to last at least
three cycles (of course, opinions differ her - I go with four). The
length of the window of interest reflects the number of cycles that can
be estimated. For source reconstruction, make sure cfg.toilim is as long
as the number of cycles you want to estimate, e.g. when using cfg.foi =
10, then cfg.toilim(2)-cfg.toilim(1) should be at least 400ms when you
want an estimate of 4 cycles.

Hope it helps!
Best,
Jörn

On 3/27/2012 12:46 PM, Ion Lavado wrote:
> Thank you Stephan, just one thing, why i need a time window big
> enough? so i'm just studying the frequency.
>
> Best,
>
> Mikel
>
> 2012/3/27 Stephan Moratti <smoratti at psi.ucm.es
> <mailto:smoratti at psi.ucm.es>>
>
>     Dear Mikel,
>
>     I understand that you should do a mtmfft with a center frequency
>     of interest (in your case cfg.foil = [11 11]) and then you do a
>     frequency smoothing of 3 Hz (cfg.tapsmofrq = 3). Of course you
>     need a time window big enough to allow such a smoothing. The CSD
>     should represent a frequency window of 8 to 14 Hz then. Then you
>     put the result of the mtmfft into the dics beamformer.
>
>     Best,
>
>     Stephan
>
>     El 27/03/2012, a las 11:19, Ion Lavado escribió:
>
>>     Hello, i'm having problems trying the beamformer analysis with a
>>     frequency band of interest. I would like to have for example an
>>     interval like [8 14] Hz. My concept problem is about the
>>     cfg.frequency and cfg.tapsmofrq  values. (it must be a escalar,
>>     but i want a frequency band).
>>
>>     %FREQ ANALYSIS
>>     cfg = [];
>>     cfg.method    = 'mtmfft';
>>     cfg.output    = 'powandcsd';
>>     cfg.tapsmofrq = 5;
>>     cfg.foilim    = [8 14];
>>     freq_words_alpha = ft_freqanalysis(cfg, words);
>>
>>
>>     %SOURCE ANALYSIS
>>     cfg              = [];
>>     cfg.frequency    = 11;
>>     cfg.lambda=0;
>>     cfg.method       = 'dics';
>>     cfg.coordsys       = 'neuromag';
>>     cfg.projectnoise = 'yes';
>>     cfg.channel   = {'MEG'};
>>     cfg.grid         = grid;
>>     cfg.reducerank      = 2;
>>     cfg.vol          = vol02;
>>     vol02=ft_convert_units(vol02,'cm');
>>     source_pre11  = ft_sourceanalysis(cfg,  freq_words_alpha );
>>
>>
>>     I use a cfg.frequency of 11 but i don't have really clear what
>>     i'm doing as a get the same result using cfg.frequency=11 but a
>>     freqanalysis in [10 12]. I would be very grateful if someone
>>     could solve my doubt.
>>
>>     Best wishes,
>>
>>     Mikel
>>     _______________________________________________
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>>     fieldtrip at donders.ru.nl <mailto:fieldtrip at donders.ru.nl>
>>     http://mailman.science.ru.nl/mailman/listinfo/fieldtrip
>
>
>     ________________________________________________________
>     Stephan Moratti, PhD
>
>
>     Departamento de Psicología Básica I
>     Campus de Somosaguas
>     28223 Pozuelo de Alarcón (Madrid)
>     Spain
>
>     and
>
>     Center for Biomedical Technology
>     Laboratory for Cognitive and Computational Neuroscience
>     Parque Científico y Tecnológico de la Universidad Politecnica de
>     Campus Montegancedo
>     28223 Pozuelo de Alarcón (Madrid)
>     Spain
>
>
>     email: smoratti at psi.ucm.es <mailto:smoratti at psi.ucm.es>
>     Tel.: +34 679219982 <tel:%2B34%20679219982>
>
>
>
>     _______________________________________________
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>     http://mailman.science.ru.nl/mailman/listinfo/fieldtrip
>
>
>
>
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--
Jörn M. Horschig
PhD Student
Donders Institute for Brain, Cognition and Behaviour
Centre for Cognitive Neuroimaging
Neuronal Oscillations Group

P.O. Box 9101
NL-6500 HB Nijmegen
The Netherlands

Contact:
E-Mail: jm.horschig at donders.ru.nl
Tel:    +31-(0)24-36-68493
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