EEG sensor registration problem with NexStim digitization

Discussions
1

Hello,

We are having difficulty registering our EEG sensors to the MRI head reconstruction. The electrodes appear to be in correct position to one another but they are flipped in a strange way relative to the imported MRI volume. The attached image EEG_3D_Subject14.jpg illustrates the problem. We suspect this is due to differences in the two xyz coordinate systems (NextStim and brainstorm) but haven’t been able to find the transform to align the sensors to the scalp. This exact problem occurs whether we use the freesurfer reconstruction or the original dicoms for generating the head model and surfaces. Moreover the problem persists even when we have Nasion, RPA and LPA positions explicitly labeled in the digitization. The electrode positions are being digitized using NextStim brain navigation system and can be exported in either EMSE .elp format (as in DigitizationEMSE.txt and DigTest2.txt attached) or in a NextStim coordinate system (I believe the scan volume coordinates) as in DigTest3.txt attached. The EMSE digitization is what we used to generate the attached image. The other export option in volume coordinates are totally off and lead to sensors way out in space. Do you have any recommendation for how to transform digitizations from NextStim into brainstorm or a good work around for this issue? We would greatly appreciate any help you can provide.

Best regards,
Benjamin Baird
Postdoctoral Research Fellow
University of Wisconsin - Madison

2

Dear Benjamin,

I fixed the import of the EMSE format: it was not using correctly the fiducial positions to align the electrodes on the scalp.
Now you should be able to import directly the DigitizationEMSE.txt file directly.
I also modified the way the automatic scaling was done, now it asks for a confirmation of the scaling factor before applying it.

You’re not going to get anything done (automatic registration or scaling) from the files DigTest2.txt and DigTest3.txt because you these operations are executed only of full EEG caps (>10 electrodes).
If you have full versions of them, your should be able to import them now with the format “EEG: ASCII: Name, XYZ”.

Please update Brainstorm and try again (menu Help > Update Brainstorm).

Cheers,
Francois

3

It appears to be working, thank you!

-Ben