The channel labels and the channel positions of the new cap are different from the old cap. To build the head model for the source localization or to remove the eyeblinks used in the method, we should update the sensor location file in the data set. EEGo has the .elc file of each eeg channel location, and the .elc file included channel labels and the 3D location (X, Y, Z) of each channel.
Since we want to build the head model in spm, our 3D coordinates of EEG electrode should be matched with the MNI coordinates in the spm toolbox. We need to co-register our EEG sensor locations with the MNI coordinates, and normally we used 'nas', 'lpa' and 'rpa'. When we add these three coordinates, we should make sure that the two systems shared the same definitions of X, Y, Z. We need to rotate the X, Y in the eego file, which we just replace the X as Y, while Y as X. Then we added the coordinates in the .elc file.
The .elc file can be edited using Excel (windows system), just drag the .elc file to Excel, and then add three more rows of nas, lpa and rpa coordinates. Similarly, add these labels too. After the change, directly save the file in Excel. (Note: this only can be done in the windows system).
The eego new cap location file should 'NA-261_SPM.elc', and should be put under the folder spm12/EEGtemplate/.
These are the definitions in the eeglab or .elc file.
- The X-axis points towards and goes through the nasion
- The Y-axis points approximately towards the ‘LPA,’ orthogonal to the X-axis
- The Z-axis points from inferior to superior, orthogonal to X and Y
https://www.fieldtriptoolbox.org/faq/coordsys/
Details of the MNI coordinate system
The Montreal Neurological Institute coordinate system is comparable to, but not exactly the same as the Talairach-Tournoux coordinate system. Rather than being based on a single specimen, it is the result from spatially transforming and averaging MRI scans of many subjects.
- The origin of the MNI coordinate system is the anterior commissure
- The X-axis extends from the left side of the brain to the right side
- The Y-axis points from posterior to anterior
- The Z-axis points from inferior to superior
