[FieldTrip] combining magnetometers and planad gradiometers for analysis

[Elena Orekhova]

Dear Michael,
I still have not resolved this problem and do not know whether  is specific for my data(or program bag)
or the other Neuromag users also had it.
Do you use Neuromag?

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: Thursday, June 09, 2011 5:06 PM
To: Email discussion list for the FieldTrip project
Subject: Re: [FieldTrip] combining magnetometers and planad gradiometers for analysis

Hi Elena,

disregard my last email, I overlooked the att.

Michael


________________________________
Von: "Elena Orekhova" <Elena.Orekhova at neuro.gu.se>
Gesendet: Jun 2, 2011 8:03:16 PM
An: "Email discussion list for the FieldTrip project" <fieldtrip at donders.ru.nl>
Betreff: Re: [FieldTrip] combining magnetometers and planad gradiometers for analysis

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Dear Michael,

I did normalize the data
(sourcepst.avg.nai=sourcepst.avg.pow./sourcepre.avg.pow)
as it was suggested in the tutorial.
I used the same scales in all plots for comparison purposes. If  automatic scaling is used the result for MAG+GRA does not look any better (see attachment).

>What you could do as a workaround is to average the separate results with a weighting per voxel and  that >is corresponding to the squared norms of the leadfields for the respective modalities for a given voxel , this >would guarantee equal amounts of backprojected noise from both modalities (if I'm not mistaken).

Thank you for your suggestion.  I may use it as a last resort.  Hopefully, there is still a way to get more optimal solution using both GRA and MAG in the same analysis...


Elena
________________________________
From: fieldtrip-bounces at donders.ru.nl [fieldtrip-bounces at donders.ru.nl] on behalf of Elena Orekhova [Elena.Orekhova at neuro.gu.se]
Sent: Thursday, June 02, 2011 6:22 PM
To: Email discussion list for the FieldTrip project
Subject: Re: [FieldTrip] combining magnetometers and planad gradiometers for analysis


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Dear Michael,

I have tried to multiply the leadfield by -1 as you suggested:

for i = 1 : size (grid.leadfield, 2)
    grid.leadfield{i}(3:3:306, :) = -1*grid.leadfield{i}(3:3:306, :);
end

This had no effect on the 'lcmv' output. I attached the pictures for ‘GRA only, ’MAG only’ and ‘GRA + MAG’

In this experiment I measured evoked field in response to  the unilateral (left) click.
The source is expected to be in the right superior temporal cortex.  This is the case  for  ‘GRA only' and ’MAG only’ datasets. The combined sensors give meaningless result.

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: Wednesday, June 01, 2011 4:04 PM
To: Email discussion list for the FieldTrip project
Subject: Re: [FieldTrip] combining magnetometers and planad gradiometers for analysis


Dear Elena,

could you give the following a try: invert (*-1) the leadfileds for one of the two sensor types. Let me know what happens.
I would also be interested in taking a look at the results - maybe you could sent images off-list: Michael.Wibral <at> web.de.

Michael



________________________________
Von: "Elena Orekhova" <Elena.Orekhova at neuro.gu.se>
Gesendet: Jun 1, 2011 12:23:29 PM
An: "fieldtrip at donders.ru.nl" <fieldtrip at donders.ru.nl>
Betreff: [FieldTrip] combining magnetometers and planad gradiometers for analysis

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Dear  fieldtrippers,
This message is mainly for Neuromag users.

When I do  'lcmv' beamforming analysis separately on planar gradiometers or magnetometers, I get quite meaningful results.
If I combine the two types of sensors without weighting, the result is meaningless.
Apparently, the algorithm does not take care of different scales and units of the GRA and MAG measurements.  Does anybody know how to deal with this problem?


Elena




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