subject too low in ctf scanner

[Robert Oostenveld]

Hi Tom

The default behaviour of the prepare_dipole_grid helper function for
sourceanalysis in case of MEG is to make a 3D dipole grid that covers
the helmet ("~30x30x30cm wide box"), subsequently detect points that
are inside the head (actually inside the volume conductor), and then
reduce the size of the 3D grid to a "narrow box" that tightly fits
around the brain. So if the subject is seated very low compared to th
edewar, then indeed the lower part of his brain might have been
skipped for the 3D grid generation.

You can also start with your own specification of the 3D grid, using
cfg.xgrid/ygrid/zgrid. The grid is specified in individual subjects
headcoordinates, i.e. in cm in the CTF case, which means that the
position of the head relative to the dewar does not matter. Something
like this should do the trick

cfg.grid.xgrid = -12:14  % back to front
cfg.grid.ygrid = -12:12  % right to left
cfg.grid.zgrid = -2:14   % bottom to top

if you then also specify cfg.grid.tight=yes, then you'll still get a
nice tight/narrow box without too many points that are outside the head.

best regards,
Robert



On 9 Feb 2009, at 8:02, Tom Holroyd wrote:

> When the subject is too low,
> the source analysis cuts off
> any activity below the dewar.
> Radial gradiometers can pick
> up sources below the sensor
> but Fieldtrip clips them.
>
> Is there an easy way to extend
> the grid so that source analysis
> can go below the sensor?
>
> ----------------------------------
> 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.