forward modelling without anatomical info

[Nathan Weisz]

hi,

on the fieldtrip FTP there is a standard BEM (standard_vol.matr). i  
think you could use the skull part as pseudo-headshape:
vol.bnd(1).pnt

this also fits with the standard MRI.

there may be some transformation you have to apply to these  
"headshape" points so that the head fits nicely into the sensor array.
then you can use prepare_localspheres and specify a lower number in  
cfg.spheremesh (perhaps something like 50; default is 4000).

this may yield a crude but satisfactory solution if you are unable to  
get MRIs or individual headshapes.

i'd be interested in the outcome.

all the best,
nathan




On 07.01.2008, at 16:59, thomas gilbertson wrote:

> dear jan mathijs,
>
> i have been generating a single shell model from the segmented mri.  
> the localisation proceedure is for a tactile stimulus so when i see  
> a contralateral S1 response i have some reason to belive that the  
> algorithm is making some sense. I get the impression from your reply  
> that there is no easy way to get around this lack of anatomical data  
> and that i may just have to go and get the mri's done for this  
> couple of subjects!
>
> thanks for your thoughts on this,
>
> tom.
>
>
> On 1/7/08, Jan Mathijs Schoffelen <jan.schoffelen at fcdonders.ru.nl>  
> wrote:
> Dear Tom,
>
> As far as I know, the 'ctf-format' standardised mri does not exist.  
> Creating an anatomically accurate forward model, while the actual  
> anatomical information is lacking, is tricky business. What kind of  
> forward model do you actully use? The multiple sphere model based on  
> the ctf-shape files? If so, you could try to come up with an  
> 'averaged' multipsphere model (thus bypassing the need for a  
> standardised MRI), although this is not trial.  However, I wonder  
> whether this would yield better results than just taking an  
> 'averaged' single sphere model.
> I wonder what you mean by your statement that localisation is much  
> more accurate when using ctf-formatted MRIs. Do you  know where your  
> activation should end up then?
>
> The function beamformer_pcc is as far as I know still in its  
> experimental stage and only for use within the FC Donders Centre.  
> However, beamformer_dics shares most of the functionality, but  
> obviously not all of it.
>
> Regards,
>
> Jan-Mathijs
>
> ----------------------------------
> 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.
>
> http://listserv.surfnet.nl/archives/fieldtrip.html
>
> http://www.ru.nl/fcdonders/fieldtrip/
>

--------------------------------
Dr. Nathan Weisz

INSERM - Unité 821
Dynamique cérébrale et cognition
Centre Hospitalier Le Vinatier, Bâtiment 452
95 Boulevard Pinel
69500 Bron, France

Tel: ++33 - (0)4 - 7213 8915
Email: weisz at lyon.inserm.fr
Chat-AV: nathanweisz at mac.com
Homepage: http://web.mac.com/nathanweisz
Neurotree: http://neurotree.org/neurotree/tree.php?pid=8692

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