One of the best databases for focal epilepsy research is the Epilepsiae database of over 100 annotated datasets of inpatient intracranial and scalp electrode recordings from individuals suffering from epilepsy. It is a fantastic resource. However, it is not clear how to interpret the intracranial electrode coordinates given the current state of the Epilepsiae documentation. According to the paper introducing the the database:
Ihle, M., Feldwisch-Drentrup, H., Teixeira, C. A., Witon, A., Schelter, B., Timmer, J., & Schulze-Bonhage, A. (2012). EPILEPSIAE–A European epilepsy database. Computer methods and programs in biomedicine, 106(3), 127-138.
the electrode coordinates should be in MNI space.
It turns out this is not exactly the case as confirmed by Dr. Matthias Dümpelmann, the corresponding author on the above paper. The issue is that MNI coordinates use the anterior commissure as the origin and the Epilepsiae coordinates use the lower, right, posterior corner of the MRI data block as the origin. To convert the Epilepsiae coordinates to MNI coordinates, do the following:
- Get the Epilepsiae coordinates for an electrode (e.g., FR_1096’s HL1 is at 112, 127, 40)
- Subtract the coordinates of the anterior commissure (AC) from #1. Because all patient MRIs have been non-linearly warped to match that of the MNI brain, the AC is always at 91, 127, 73.
- Flip the sign of the first (Left-Right) coordinate. For example, for FR_1096 HL1 we get final MNI coordinates of -21, 0, -33.
Now you can plot the electrode locations on the MNI brain or use an MNI atlas to identify the anatomical locations (e.g., hippocampus).