Volume conduction model for EEG DICS beamforming

[Robert Oostenveld]

Hi Jared,

On 29 Jun 2007, at 3:31, Jared Van Snellenberg wrote:
> I'm currently working on a few DICS beamforming analyses for an EEG
> dataset
> I have, and I have a couple of questions regarding obtaining a good
> volume
> conduction model to include in the cfg.vol input to sourceanalysis.
>
> I do not have MRIs or headshape information for my subjects, but of
> course
> I have electrode position information in spherical coordinates
> (stored in
> an elec structure imported from a BESA .elp file).

As far as I know, the BESA elp format only contains the two angles
that define the electrode position in spherical coordinates (i.e. see
http://en.wikipedia.org/wiki/Spherical_coordinates) to project them
onto a spherical head model. The 3D carthesian coordinates would be
more usefull and realistic (i.e. an x, y, and z value for each
electrode).

> It's not clear to me how to generate a volume conduction model with
> this
> information, so any guidance here would be greatly appreciated.
> Ideally, I
> would like to use a 4-shell model.

You can use a 4-shell concentric sphere model. That is specified using
vol.r = [r1 r2 r3 r4] % radii of the shperes
vol.c = [c1 c2 c3 c4] % conductivit within each sphere
vol.o = [x y z]       % optional, origin of the sphere. Default is to
assume [0 0 0]

You can fit the radius of the outermost sphere to the electrodes. For
the elp files I would assume that the origin of the sphere would be
[0 0 0], which means that you do not have to specify that.

The standard spherical model used in BESA is
vol =
     r: [71 72 79 85]
     c: [0.3300 1 0.0042 0.3300]
     o: [0 0 0]

> The FIELDTRIP documentation indicates
> that this model is implemented, but I can not seem to find the
> relevant
> function (and prepare_singleshell requires headshape information or
> an nx3
> sensor array--I presume in grid coordinates--that I do not have).

The prepare_localspheres and prepare_singleshell are indeed for MEG
volume conductors. They can use a polhemus scanned surface of the
head, or a large set of points on the brain (from an MRI) or they can
use a segmented anatomical MRI.

> Finally,
> all of the relevant prepare_XXX functions seem to indicate that
> they are
> specifically for MEG data, so I wanted to make sure that whatever
> approach
> I use is appropriate for EEG electrodes rather than MEG sensors.

You can also use a standard realistic EEG model using the boundary
element method (BEM), which is based on the SPM/MNI template brain.
That is not included by default in the fieldtrip release version but
I have just copied it as a seperate file to our ftp server. You can
find it on ftp://ftp.fcdonders.nl/pub/fieldtrip/standard_BEM.zip

best regards,
Robert

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The aim of this list is to facilitate the discussion 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/fcdonders/fieldtrip.