Problem while using prepare_leadfield with parameters cfg.resolution and cfg.inwardshift

[Robert Oostenveld]

Hi Ingmar

Although your question/problem is not completely clear to me, I
suspect it to be related to

http://www2.ru.nl/fcdonders/fieldtrip/doku.php?
id=fieldtrip:documentation:frequently_asked_questions#why_is_there_a_rim
_around_the_brain_for_which_the_source_reconstruction_is_not_computed

In general fieldtrip tries to make a dipole grid that is just inside
the brain but as close as possible to the brain surface. Some volume
conduction models are not fitted to the brain surface however, but to
the skin surface. In those cases the cfg.inwardshift option tries to
get an estimate of the brain surface from the skin surface, in order
to achieve teh same tight fit of the dipole grid in the brain and to
prevent the solution to be computed for dipoles in teh skull or skin.

However, with a low resolution and a grid that fits exactly in the
brain you tend to get the problem as outlined in the FAQ. Therfore it
can be usefull to play with the cfg.inwardshift parameter. You can
set it to a negative number, i.e. inwardshift=-2.5, which results in
a 2.5cm _outwardshift_ of the source compartment boundary.

I hope this helps, if not then please ask again with more
specifications of your analysis (what kind of MEG, what kind of
volume model, etc).

best regards,
Robert


On 13 Feb 2008, at 15:36, Ingmar Schneider wrote:
> Hello everybody,
>
> I am currently trying to analyze my MEG data with the DICS beamforming
> technique. While preparing my leadfield grid via the
> prepare_leadfield.m
> function I stumbled across a problem I don't understand. Every time
> I try to
> decrease the inwardshift to suit the leadfield upon my anatomy data
> and at
> the same time increase the resolution the resulting grid is even
> smaller,
> than without specification.
>
> For example applying the following parameters reduces the size of the
> resulting grid instead of increasing its volume:
> cfg.inwardshift = -1.5; %Increasing the grid size to cover the anatomy
> cfg.resoltion = 0.5;    %[cm]
>
> Without the cfg.resolution parameter a reduction of cfg.inwardshift
> results
> in the desired bigger leadfield grid which eventually fits upon the
> MRI
> data. Are these factors intertwined in the calculations so that one
> effects
> the other in the course of calculation?
>
> I hope someone can help me solve this little problem and am
> grateful for any
> kind of hint.
>
> Regards,
> Ingmar
>
> ----------------------------------
> 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.
>

----------------------------------
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.