[FieldTrip] template brain/standard EEG-Electrodes

[Nathan Weisz]

hi,

somewhere on the firledtrip FTP there is a standard_BEM. i got mine from EEGlab, but it's fieldtrip stuff. it's ~11 MB so i won't send it over the list :-)

anyway: this is a headmodel created by segmentation of a MNI-brain. if you follow the fieldtrip tutorial on DICS this contains the "vol"-structure you need.
the only - and sometimes not trivial - issue you have is to get your electrodes aligned / co-registered onto the head surface. you have different options (i guess you don't have individual MRIs otherwise you wouldn't be asking), e.g.:
- there are standard electrode positions in MNI space for 10/05 electrode placements somewhere in fieldtrip. you may find all / most electrodes of you EEG in this file. that's fast and no headache.
- your EEG vendor may have such a file specific for your cap. we got one from our EEG supplier (ANT). even better than the option above, because then you are sure to have all electrodes (especially when the cap deviates from 10/20 or 10/05 placement).
- digitize the electrode-positions in one subject and co-register it to the MNI template brain manually. this option is probably a pain since the head of any individual you choose won't fit exactly the MNI template. so you will have to play around until the electrodes are as much as possible on the head surface. [unless: if you can also get an MRI for that subject: then co-register electrodes with individual MRI; normalize individual MRI to MNI-template; take the resulting transformation matrix to recalculate your elec.pos field in MNI coordinates ... that would also be an easy option]

in any case once your electrode data, headmodel and MRI (latter not strictly needed for the sourceanalysis, but you will want it for visualization) are in the same coordinate sytsem, then you can proceed as described in the tutorial. there is really no difference anymore whether you have individual or standard stuff, i.e. the sourceanalysis won't care as long as it gets the structures it wants.

a word of consolation: if you want to use the standard headmodel / brain, then you need to solve this issue only one time :-)

good luck,
n


On 17.03.2011, at 08:12, Lydia Elshoff wrote:

> hello!
> 
> We are a research group at the Neuropaediatrics Department in Kiel and started working with EEG Analysis in fieldtrip. We now face several questions: Could you please tell us how to or send us a matlab code for the using template brain and the standard EEG-Electrodes in DICS analysis in fieldtrip? Or perhaps show an example???
> Thank you very much!!!
> 
> Lydia Elshoff
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