MNI contact coordinates deviate from 3.5 mm spacing in two SEEG patients despite straight shafts

We compared published MNI152Lin contact coordinates in OpenNeuro ds005670 v1.0.0 with the dataset README's stated 3.5 mm center-to-center spacing for Sinovation SDE-08 electrodes. The analysis included two patients and used electrodes.tsv coordinates, without signal data. A plan was written and hashed before the electrode files were downloaded. In the preregistered analysis, only 40.6% of 219 adjacent pairs in sub-01 and 43.9% of 171 pairs in sub-02 were within 0.5 mm of 3.5 mm. Median spacing error was +0.48 mm in sub-01 (criterion failed) and +0.07 mm in sub-02 (criterion passed). All 29 shafts with at least four contacts met the straightness criterion (RMS distance from a fitted 3D line <=0.5 mm). Labels DC and EKG had repeated placeholder coordinates; excluding them in a post hoc sensitivity analysis raised the within-0.5-mm fractions to 42.8% and 46.0%, respectively, without changing the conclusions. Eight pairs were more than 7 mm apart. Thus, for these data, adjacent MNI contact distance was not a reliable estimate of the stated physical spacing, despite straight shafts. Possible causes include nonlinear MNI warping, CT localization error, and patient brain size; they cannot be distinguished because native-space coordinates are not published. Any implication for RAM coordinates is conditional: RAM data were not tested. The result is descriptive for two patients, one center, and one electrode model, with no group inference.

stereo-EEG · SEEG · electrode coordinates · MNI space · contact spacing · OpenNeuro