forward models

[Robert Oostenveld]

On 6 Dec 2006, at 17:34, Sameer Aroon Walawalkar wrote:
> I am learning how to use fieldtrip to do DICS (voxel beamforming)
> on my data.  To do this, I need leadfield using some forward
> model.  How do I calculate the forward model for my setup?  My data
> is collected using Elekta Neuromag (102 x 3 channels). Also, I have
> converted it to .mat format, have extracted the trials etc and will
> be passing these files to fieldtrip functions.

There is nothing specific about the forward models in fieldtrip for
CTF, except that fieldtrip can easily read CTF head models. See the
following FAQ
http://www2.ru.nl/fcdonders/fieldtrip/doku.php?
id=fieldtrip:documentation:frequently_asked_questions#what_kind_of_volum
e_conduction_models_are_implemented

There are the following options for neuromag.

1) You can use the headmodel and leadfields from the neuromag
software (requires the meg_calc toolbox). For that, you specify
cfg.hdmfile=your_fif_file_with_the_headmodel.

2) The alternative is to use prepare_singleshell or
prepare_localspheres (that can also fit a single sphere to teh
anatomy). Both functions require a correctly realigned individual
anatomical MRI of your subject (you can use volumerealign to help you
with the coordinate system specification). The output of these
prepare_xxx functinos is put into cfg.vol (the cfg.vol applies to
prepare_leadfield, sourceanalysis or dipolefitting).

3) Finally, you can specify a simple single-sphere model using
cfg.vol.r = radius
cfg.vol.o = origin
also this applies to prepare_leadfield, sourceanalysis or dipolefitting

> I am sorry that this is not a technical question about fieldtrip,
> but all the information on fieldtrip help seems to be about CTF
> based forward models and seem to use MRI scans.

Volume conductor models require geometrical information, and that
geometrical information is obtained from teh anatomical MRI. If you
do not have an anatomical MRI, then you do not know how the brain of
your subject was located with respect to the MEG sensors. You still
can use a single sphere model, but you would have to estimate the
radius and the position of the origin of the sphere. Look at
http://www2.ru.nl/fcdonders/fieldtrip/doku.php?
id=fieldtrip:documentation:frequently_asked_questions#how_are_the_differ
ent_head_and_mri_coordinate_systems_defined
for more informatino about coordinate systems (which you should be
aware of if you want to manually specify a volume conduction model).

best regards,
Robert