Dear Chris,
The template locations are stored already in MNI space under
spm/MEEGtemplates/ext1020.sfp . You could save ext1020.sfp under a
different name, remove 'spm' prefixes from the fiducial names and then
find at least 3 electrodes whose positions for your lab you can find
on the MNI template image. You can then load the file as individual
electrode locations and use those electrodes as fiducials with the
'type' option to enter the corresponding MNI coordinates. You might
want to say 'no' to 'use headshape' and see if that works better.
Alternatively, if you have a 3D digitizer you could digitize one
subject with the cap put the way you put it + fiducials and use that
as individual electrode positions for all subjects. EEGLAB and
Fieldtrip have some tools for manual alignment that you might want to
look at.
Best,
Vladimir
On Tue, Oct 25, 2011 at 5:57 PM, Chris Brown
<[log in to unmask]> wrote:
> Dear SPMers,
>
> I am performing EEG source localisation using the batch tool. We did not measure electrode positions or MRIs so I'm relying on the defaults (61 scalp electrodes, extended 10-20 system). However, the main source I have estimated for a well-known ERP (the P2 peak of the laser-evoked potential) is too deep in the brain by at least 2-4cm, appearing in the corpus callosum when it should be in the mid/posterior cingulate. My guess is that there is an issue with the electrode positions, because the way we position our caps appears different to how the electrodes are positioned in MNI space according to the images created by SPM. SPM appears to automatically create coordinates in MNI space. Is there a way to manually adjust the way this step is performed?
>
> Many thanks,
> Chris
>
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