Variability of EEG electrode positions and their underlying brain regions: visualising gel artifacts from a simultaneous EEG-fMRI dataset

We investigated the between-subject variability of EEG electrode placement from a simultaneously recorded EEG-fMRI dataset. Neuro-navigation software was used to localise electrode positions in xyz and MNI space, made possible by the gel artifacts present in the structural MRI images. To assess variation in the brain regions directly underneath each electrode, we used both raw MNI coordinates and labels from the Harvard-Oxford Cortical atlas. In a sample of 20 participants, the mean standard deviation of electrode placement was 3.94 mm in x, 5.55 mm in y, and 7.17 mm in z, with the largest variation in parietal and occipital electrodes. In addition, the brain regions covered by electrode pairs was not always consistent; for example, the mean location of electrode P07 was mapped to BA18, whereas P08 was closer to BA19. Further, electrode C1 was mapped to the left primary motor cortex, whereas C2 was closer to right pre-motor cortex. Overall, the results emphasise the variation in electrode positioning that can be found even in a fixed cap, potentially caused by between-subject differences in brain morphology. We present a relatively simple method for approximating the location of electrodes in a simultaneous EEG-fMRI data set with accompanying analysis code, and suggest that researchers check the regions underlying their EEG ROIs to improve the generalisability and reliability of their neuroimaging results.