coordinates outside brain volume

[jan-mathijs schoffelen]

Dear Vasan,

If you don't fully specify a 'grid' (i.e. the locations of your
virtual channels and an assignment whether they are inside the brain
volume or not) fieldtrip tries to automatically detect whether the
locations (either specified by you, or specified by some semi-clever
algorithm) fall inside the volume conductor model. For a single sphere
which is fitted to the headsurface (or to the inside of the skull)
this could lead to many points being excluded.
I think generally there are two ways to deal with it:

1 either explicitly specify your virtual channel locations to be
'inside'. In other words: provide fieldtrip's ft_sourceanalysis with a
cfg structure containing a grid, in which you state grid.inside =
1:size(grid.pos,1) and grid.outside = [];
2 alternatively, as already mentioned to you,  inflate the single
sphere, because for a single sphere volume conductor it's the origin
that counts.

Alternatively, if you have the anatomical scan of your subject, use a
volume conductor model which is more 'realistic', taking the geometry
of the inside of the skull into account.

Cheers,

Jan-Mathijs


On Feb 15, 2010, at 1:58 PM, Rajamoni Nadar, Sreenivasan (NIH/NIMH)
[E] wrote:

> Dear Users,
>
> I am using the FieldTrip to extract single trial source signals
> using the CTF coordinates. Although I have been successful but for
> some coordinates the Fieldtrip is giving error as coordinates are
> outside the brain volume. The same coordinates are very much inside
> brain volume and the CTF software does not report error to extract
> the virtual source signals. An expert suggested to "inflate" the
> sphere model so that coordinates are within brain volume to do the
> source estimation. Anybody has a better idea to deal the problem in
> FieldTrip?
>
> Thank you
>
> Vasan
>
> ----------------------------------
> 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/neuroimaging/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: 0031-24-3668063

----------------------------------
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/neuroimaging/fieldtrip.