Volume conduction model for EEG DICS beamforming

[Robert Oostenveld]

On 6 Aug 2007, at 21:59, Jared Van Snellenberg wrote:

> I'm using the BEM head model you supplied, and have a quick
> question about it. From plotting the results of this approach, the
> grid appears to only extend as low as the level of the anterior
> commisure. Is this intended? If so, I presume it's because of the
> difficulty in localizing deep sources? Thanks in advance.

Hi Jared,

There is no reason not to let the grid extend all the way down to the
bottom of the cerebellum. At the lower locations it will be less
sensitive and more blurred, but that should not stop you from looking
there. If you use cfg.grid.xgrid='auto' etc. for sourceanalysis/
prepare_leadfield, then the grid is determined from a box that
tightly fits the electrodes. Probably that is why you don't see low
grid locations, because you do not have sufficiently low electrodes.
Just do

cfg.grid.xgrid = -100:10:100 % in milimeter, since MNI coordinates
cfg.grid.ygrid = -140:10:120
cfg.grid.zgrid =  -70:10:120
cfg.tightgrid = 'yes'

and you probably should get a complete coverage (you may want to play
with the numbers to get it right). The tightgrid option makes the
bounding box of the grid as tight as possible around the brain
compartment after making the grid and after checking which locations
are in the brain. That helps in reducing the number of irrelevant/
outside-brain grid locations. So starting with a grid from
-300:10:300 in each direction would also be fine, since tightgrid=yes
ensures that the bounding box will be as tight as can be. It will
just be slower if you start with a grid that is too wide around the
brain.

Robert

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