Update: Freqstatistics Now Yields (Som e) Significant Clusters
wibral at BIC.UNI-FRANKFURT.DE
Thu Jul 2 11:50:33 CEST 2009
beamforming relies on the data (sensor) covariance matrix for the case of LCMV and other time domain beamformers and you get the focus on the desired frequency band by prefiltering
For frequency domain beamforming - like DICS - computations rely on the cross spectral density matrix - again computed at the frequencies of interest. I do not think that BESA TFC files contain that information.
That said, you can - depending on your Version of BESA and the modules included - use BESA's own beamformer (which allows for two bilateral synchronized sources to be present as an extra feature) and export the .dat file for statistics (I haven't done that myself).
But using beamforming in Fieldtrip is also straightforward and very flexible. Let me know if you need help with that.
> -----Ursprüngliche Nachricht-----
> Von: "Charles Cook" <charles.cook at ULETH.CA>
> Gesendet: 02.07.09 02:56:36
> An: FIELDTRIP at NIC.SURFNET.NL
> Betreff: Re: [FIELDTRIP] Update: Freqstatistics Now Yields (Som e) Significant Clusters
> Hi Michael,
> We are interested in determining just that, the presence of spatially and
> temporally contiguous effects that cross a certain threshold between my two
> groups (males vs. females) and within them (task 1 vs. task 2). After fully
> completing the analysis, I was able to find significant clusters in my
> within groups analysis (e.g. TF differences in male participants, task 1 vs.
> task 2, using dependent t-test) within a few discrete frequency bands. What
> we're now interested in doing is beamforming those discrete windows and
> frequencies that we have found with the cluster analysis in FieldTrip. Is
> this a feasible approach with the BESA exported TFC files? We had considered
> exporting VMP files from BESA and beamforming using Brain Voyager but
> thought that it might be simpler to do this all within FieldTrip.
> On Thu, 25 Jun 2009 12:00:31 +0200, Michael Wibral
> <wibral at BIC.UNI-FRANKFURT.DE> wrote:
> >Hi Charles,
> >thanks for the update. Cluster based statistics is exactly what the name
> says: A statistics telling you whether you have spatially and temporally
> contiguous effects that cross a certain threshold - in sum over the cluster.
> It is sometimes worth considering, whether this is what you want to test
> after all. e.g. extended effects of small effect size per electrode but
> large time/frequency extent and effects of large effect size but small
> time/frequency extent may have similar cluster statistics. The even compete
> in the sense that randomizations of the larger of the two (in total cluster
> sum) may still have larger cluster statistics than the smaller of the two,
> thus effectively rendering in non-significant. Bear in mind that the only
> thing really tested is the exchangeability of the data (which is the null
> hypothesis). That may sometimes make your results more difficult to
> interpret. You could also try cfg.correctm = 'fdr', to get classical FDR
> correction, but you may loose sensitivity in some cases.
> >One last thing: Check carefully that there is no factor that has been
> balanced over subjects (e.g. response hand) that may be resorted in the
> randomizations. For example: half of the subjects report match with the
> right hand and non-match with the left hand, the other half responding with
> an inverted assignment. This analysis setup:
> >1. violates the exchangeability hypothesis from the start (and you know!),
> but not in the way you wanted to test it - this is a serious error in
> applying randomization testing...
> >2. Consequently, it renders all other effects insignificant because the
> sorted response hand effects in the randomizations most likely exceed any
> other effect in the unrandomized data.
> The aim of this list is to facilitate the discussion between users of the FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.
The aim of this list is to facilitate the discussion between users of the FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.
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