[FieldTrip] combining magnetometers and planad gradiometers for analysis

Fri Jun 3 13:11:02 CEST 2011

Hi Stephan et al,

Any balancing coefficients (e.g. the digital weights for the bti- 
system, or ctf's 3d order gradient coefficients) are only applied to  
the leadfield if they end up in the data.grad.tra matrix. These .tra  
matrices will be compiled when reading the data header, by functions  
like XXX2grad (in fileio/private: ctf2grad, bti2grad, fif2grad).
You can check whether it is constructed using hdr.orig.projs.
If not, it may be worthwile to implement I guess.

Best,

JM

On Jun 3, 2011, at 11:00 AM, Stephan Moratti wrote:

> 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
>
>
> _______________________________________________
> fieldtrip mailing list
> fieldtrip at donders.ru.nl
> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip

Dr. J.M. (Jan-Mathijs) Schoffelen
Donders Institute for Brain, Cognition and Behaviour,
Centre for Cognitive Neuroimaging,
Radboud University Nijmegen, The Netherlands
J.Schoffelen at donders.ru.nl
Telephone: 0031-24-3614793

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