Hi all,
I'm relatively new to source localization, and noticed that for one of the subjects in this dataset I have been working with (Sub-01), their digitized fiducials are located inside the head.
As an example:
"Sub-01: "LPA : (-50.2, -0.0, -0.0) mm, Nasion : (0.0, 24.8, -0.0) mm, RPA : (65.5, 0.0, -0.0) mm"
Sub-02: "LPA : (-72.2, 0.0, -0.0) mm, Nasion : (-0.0, 114.8, -0.0) mm, RPA : (68.4, 0.0, -0.0) mm"
Sub-03: "LPA : (-78.0, -0.0, 0.0) mm, Nasion : (-0.0, 111.5, 0.0) mm, RPA : (73.5, -0.0, 0.0) mm"
The nasion y-value value is markedly lower than the other subjects. The documentation stated: "Electroencephalographic data were acquired at 1450 Hz (Nexstim eXimia EEG) from 60 channels placed according to the international 10-20 system and the manufacturer's standard cap. The ground electrode was placed on the bridge of the nose. One diagonal bipolar electro-oculogram (EOG) channel was placed around the right eye. Channels were prepared to less than 5 kiloOhms impedance. The scalp location of each channel was digitized after cap preparation and prior to recordings. We also digitized the nasion and both pre-auricular points, plus 100 additional scalp points not corresponding to any EEG sensor, to improve the quality of MRI-to-digitization co-registration.
[...]
Three anatomical fiducials were digitized for aligning the EEG with the pre-operative MRI: the nasion (lowest depression between the eyes) and the left and right ears (lowest depression between the tragus and the helix, above the tragus). This procedure is illustrated here: http://neuroimage.usc.edu/brainstorm/CoordinateSystems#Subject_Coordinate_System_.28SCS_.2F_CTF.29" but nothing else in particular about this subject.
What could this be? A shift in origin/reference? I contacted one of the authors who answered questions when the dataset was made available, and they were not sure as it had been a long time since the study was published.