Combining different MEG sensortypes

[Tolga Özkurt]

This had also been a question in my mind for a while.
Hey Michael,


This had also been a question in my mind for a while.

As you say so, magnetometers and gradiometers have different noise levels and 
obviously different units; I believe the magnetoemeters are wegihted by some 
value like "100" in Maxwell Filtering (for SSS decomposition) to avoid the 
singularity in the gain matrix. However, when I tried the same weigthing for 
beamforming algorithm in the Fieldtrip toolbox for a real data experiment, I 
could not get a good performance when I compared the localization results to the 
results obtained with "only gradiometers" or "only magnetometers". That is, 
weighting 100 was not optimal; although the results was much better than the 
lozalization result "with no weighting" at all. This means some optimal 
weighting required.

I suppose it makes sense to use the fabric noise levels of the sensors while 
weighting them. There is also a way suggested by by Henson et. al. (2009) that 
uses a Bayesian scheme to obtain optimal noise estimates, although I did not 
attempt to work into that approach yet. 


By the way, could you tell me the date and title of the "the recent paper by 
Matthew Brookes" you mentioned? It sounds like an interesting one.

Regards,

Tolga  






----- Original Message ----
From: Michael Wibral <michael.wibral at WEB.DE>
To: FIELDTRIP at NIC.SURFNET.NL
Sent: Mon, November 22, 2010 7:54:44 PM
Subject: [FIELDTRIP] Combining different MEG sensortypes

Dear Fieldtrip users (with a Neuromag system),

I have a question on how to combine the Information from the planar gradiometers 
and the magnetometers of a 306 channel Neurmag system best for beamformer weight 
computation and source time course reconstruction. Do you compute a complete 
leadfield mixing both types of gradiometers (i.e. you do an unweighted 
analysis)? Do you somehow weight the sensors for their different noise levels? 
Do you compute two sets of timecourses (one from grads, one from megnetometers)?
A related question: Do you update the leadfileds for projections that 
MAxfiltering does (like it should be done when using ICA)?

I am asking because it has been shown that the more sensors are available the 
better the time course reconstruction (a recent paper by Matthew Brookes). Hence 
it would be a pity to have to throw some of the information away.

Michael

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You are receiving this message because you are subscribed to
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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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