Beamforming with reference channels

[jan-mathijs schoffelen]

Hi Sharon,

If the results are exactly the same, I guess the references have not
been taken into account at all. Although it remains to be seen whether
you gain a lot with this approach (my preferred alternative would be
to clean the data from external noise using the references and taking
into account the correction in the forward computation), it should be
possible to do it in fieldtrip. To get it to work, you have to check
whether the necessary information is present throughout your analysis
pipeline. Importantly, when computing the covariance matrix in
ft_timelockanalysis, you have to include the references. The same goes
for the computation of the leadfields (if you use ft_prepare_leadfield
for that). If this information is still present prior to your call to
ft_sourceanalysis, you may want to check whether you have to specify
cfg.channel (to include the references, rather than 'MEG' (which may
be the default, and causes selection of close-to-the-head sensors only).
Just for the record: is it 4D-neuroimaging data you are working on?

Best wishes,

Jan-Mathijs


On Aug 2, 2010, at 2:06 PM, sharon riwkes wrote:

> Hi all,
> I tried to do source localization, after beamforming with the LCMV
> method. I did it once for all the MEG channels, and once for MEG
> channels AND the reference channels. I was hoping that using the
> reference channels will improve the localization by ignoring noise
> coming from outside of the brain.
> However, I got the exact same results for MEG+reference channels as
> for MEG alone!
> Does anyone know how to include the reference channels effectively
> in the LCMV beamforming?
> Thanks for your help,
> Sharon Riwkes.
>
> ----------------------------------
>
> 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.
>
> http://listserv.surfnet.nl/archives/fieldtrip.html
>
> http://www.ru.nl/fcdonders/fieldtrip/
>

Dr. J.M. (Jan-Mathijs) Schoffelen
Donders Institute for Brain, Cognition and Behaviour,
Centre for Cognitive Neuroimaging,
Radboud University Nijmegen, The Netherlands
J.Schoffelen at donders.ru.nl
Telephone: 0031-24-3668063


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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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