[FieldTrip] SAM beamforming on Neuromag data

[Stephan Moratti]

Hallo Nina,

Vielen Dank !!!

Stephan

El 31/05/2011, a las 09:23, Nina Kahlbrock escribió:

> Hi Stephan,
>  
> you can find the discussion list at:
> https://www.jiscmail.ac.uk/cgi-bin/webadmin?A0=NEUROMEG
>  
> Best,
> Nina
>  
> Von: fieldtrip-bounces at donders.ru.nl [mailto:fieldtrip-bounces at donders.ru.nl] Im Auftrag von Stephan Moratti
> Gesendet: Montag, 30. Mai 2011 21:27
> An: Email discussion list for the FieldTrip project
> Betreff: Re: [FieldTrip] SAM beamforming on Neuromag data
>  
> Hi Michael,
>  
> Thanks for your comments! We have just started with a Neuromag system in Madrid and we ran into the same questions. Thanks for posting your comments. Could you tell me where I can find the Neuromag discussion list, please?
>  
> Best,
>  
> Stephan
>  
> El 30/05/2011, a las 17:58, Michael Wibral escribió:
> 
> 
> 
> Hi Elena,
> 
> as far as I can see from the neuromeg discussion list and the maxfilter papers the properties of the components removed by the maxfilter do not require a leadfield update.
> If you run into rank-deficiency issue with the covariance matrix a tiny amount of regularization should fix this.
> (Note: If someone who reads this is of a different opinion, please let me know!)
> 
> 
> Michael
> 
> Von: "Elena Orekhova" <Elena.Orekhova at neuro.gu.se>
> Gesendet: May 30, 2011 4:30:34 PM
> An: "Email discussion list for the FieldTrip project" <fieldtrip at donders.ru.nl>
> Betreff: Re: [FieldTrip] SAM beamforming on Neuromag data
> 
> Thank you for this.
> 
> I have more concerns.  I applied MaxFilter to the data. Since MaxFilter reduces the rank of the covariance matrix by removing noisy components, it may influence the beamformer results.
> Is it safe to do beamforming with MaxFiltered data?
> 
> Elena
> 
> From: fieldtrip-bounces at donders.ru.nl [fieldtrip-bounces at donders.ru.nl] on behalf of Michael Wibral [michael.wibral at web.de]
> Sent: Monday, May 30, 2011 2:08 PM
> To: Email discussion list for the FieldTrip project
> Subject: Re: [FieldTrip] SAM beamformeing on Neuromag data
>  
>  
> Hi Elena,
> 
> as far as I know, the leadfield computation should be aware of the different UNITS (not only scales) of gradiometers and magnetometers. There was a problem with the sign of the leadfields but that should have been fixed.
> 
> There is one more fundamental problem however, that you should be aware of (doesn't invalidate your source analysis but bears potential for fine-tuning), which is the projection of noise:
> In beamforming the unit gain constraint guarantees that you get your source signal back with unit gain. Added on top however is neurophysiological crosstalk (minimized) and sensor noise of the sensors with the largest weights in your Beamformer (not reducible). So different sensor types willhave different (inverse) leadfield strengths, theerfore also fiofefrent source noise levels. the relative benefits of each sensor type changes from location to location, so a location (and data) dependend weighting would in principle be best.
> I am not sure if and how this is implemented if FT (Bayesian weighting would be optimal here..)
> 
> What you could do as a first step is to beam separately and compare the results.
> 
> Michael
> 
> 
> Von: "Elena Orekhova" <Elena.Orekhova at neuro.gu.se>
> Gesendet: May 30, 2011 1:08:29 PM
> An: "fieldtrip at donders.ru.nl" <fieldtrip at donders.ru.nl>
> Betreff: [FieldTrip] SAM beamformeing on Neuromag data
> 
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> Dear All,
> I try to run beamformer analysis on the auditory MEG data (Neuromag) and have basic questions.
>  
> 1.Magnetometers and gradiometers Neuromag sensors have different scales. Does the Fieldtrip take care of this difference or should I normalize the data?   It yes, how to  normalize?
>  
> 2. I would like to do SAM analysis  of evoked field and look at the time courses at ROIs  (virtual channels).  The only tutorial example I have found was for the lcmv-beamformer 
> (cfg.method = 'lcmv'; http://fieldtrip.fcdonders.nl/example/lcmv-beamformer). I am not sure 
> which parameters should I specify in ft_sourceanalysis if cfg.method = 'sam'.   
>  
>  
> I would be most grateful for any example script of this type analysis!
>  
> Regards, 
> Elena
>  
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>  
> 
> ________________________________________________________
> Stephan Moratti, PhD
> 
> see also: http://web.me.com/smoratti/
> 
> Department of Basic Psychology
> Universidad Complutense de Madrid
>  
> Centro de Tecnología Biomédica CBT,
> Universidad Politécnica de Madrid,
> 
> en la actualidad (currently at) en el
> Centro de Magnetoencefalografía Dr. Perez Modrego,
> Universidad Complutense de Madrid,
> Faculdad de Medicina,
> Pabellón 8,
> Avda. Complutense, s/n,
> 28040 Madrid,
> Spain,
> 
> email: smoratti at psi.ucm.es
> Tel.:    +34 91 394 2186
> Fax.:   +34 91 394 2294
>  
> _______________________________________________
> fieldtrip mailing list
> fieldtrip at donders.ru.nl
> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip


________________________________________________________
Stephan Moratti, PhD

see also: http://web.me.com/smoratti/

Department of Basic Psychology
Universidad Complutense de Madrid

Centro de Tecnología Biomédica CBT,
Universidad Politécnica de Madrid,

en la actualidad (currently at) en el
Centro de Magnetoencefalografía Dr. Perez Modrego,
Universidad Complutense de Madrid,
Faculdad de Medicina,
Pabellón 8,
Avda. Complutense, s/n,
28040 Madrid,
Spain,

email: smoratti at psi.ucm.es
Tel.:    +34 91 394 2186
Fax.:   +34 91 394 2294

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