[FieldTrip] combining magnetometers and planad gradiometers for analysis

[Stephan Moratti]

Hi Michael y Elena,

I am following your discussion with great interest as I have just started to work with Neuromag data. As FT uses the MNE toolbox to read the data I assume that the SSP vectors are applied to the data during reading the data. However, the SSP vectors are stored in "data.hdr.orig.projs". However, when I go on with averaging in the ERF from timelockanalysis I cannot find this info. So my question is, does FT apply the SSP vectors later on also to the leadfield when calculating it? I assume that not, as the SSP vectors are not passed on further. How can I apply the SSP vectors to the leadfield to get correct results?

Best and thanks,

STephan

El 02/06/2011, a las 19:20, Michael Wibral escribió:

> Hi Elena,
> 
> I assume that you were well aware of the difficulties in beamforming evoked activty and baseline normalisation and these things, your results really look that way. I mus say I am quite puzzled by the results - could it be a color scaling problem in the plot? I think this mmay be a problem because most of your plot is a peak power, only a tiny corner seems to be in a low-amplitude range.
> 
> 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).
> 
> Michael
> PS: I am away for a couple of days, but after that, I'll be happy to resume this discussion.
> 
> 
> Von: "Elena Orekhova" <Elena.Orekhova at neuro.gu.se>
> Gesendet: Jun 2, 2011 6:22:03 PM
> An: "Email discussion list for the FieldTrip project" <fieldtrip at donders.ru.nl>
> Betreff: Re: [FieldTrip] combining magnetometers and planad gradiometers for analysis
> 
> @font-face { font-family: "\FF2D \FF33 \660E \671D "; }@font-face { font-family: "Cambria Math"; }@font-face { font-family: "Calibri"; }@font-face { font-family: "Cambria"; }p.MsoNormal, li.MsoNormal, div.MsoNormal { margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: Cambria; }.MsoChpDefault { font-family: Cambria; }div.WordSection1 { page: WordSection1; }
> 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
> 
> @font-face { font-family: "Arial"; }@font-face { font-family: "Times"; }@font-face { font-family: "\FF2D \FF33 \660E \671D "; }@font-face { font-family: "Cambria Math"; }@font-face { font-family: "Cambria"; }p.MsoNormal, li.MsoNormal, div.MsoNormal { margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: Cambria; }p { margin-right: 0cm; margin-left: 0cm; font-size: 10pt; font-family: Times; }.MsoChpDefault { font-family: Cambria; }div.WordSection1 { page: WordSection1; }
> 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
> 
>  
>  
> _______________________________________________
> 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/

Universidad Complutense de Madrid
Facultad de Psicología
Departamento de Psicología Básica I
Campus de Somosaguas
28223 Pozuelo de Alarcón (Madrid)
Spain

and

Center for Biomedical Technology
Laboratory for Cognitive and Computational Neuroscience
Parque Científico y Tecnológico de la Universidad Politecnica de Madrid
Campus Montegancedo
28223 Pozuelo de Alarcón (Madrid)
Spain


email: smoratti at psi.ucm.es
Tel.:    +34 679219982


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