[FieldTrip] SAM beamforming on Neuromag data

[Nina Kahlbrock]

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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fieldtrip at donders.ru.nl
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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

 

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