High-density Electric Source Imaging of interictal epileptic discharges: How many electrodes and which time point?

•Down-sampling of 257- to 204-channel EEG by removing cheek and neck electrodes can improve accuracy of EEG source localisation.•This may be due to both higher artefact load in caudal channels and insufficient biophysical modelling of inferior head areas.•In comparison to earlier time points or the spike peak, 50% of the averaged spike’s rising phase provided most accurate results. To assess the value of caudal EEG electrodes over cheeks and neck for high-density electric source imaging (ESI) in presurgical epilepsy evaluation, and to identify the best time point during averaged interictal epileptic discharges (IEDs) for optimal ESI accuracy. We retrospectively examined presurgical 257-channel EEG recordings of 45 patients with pharmacoresistant focal epilepsy. By stepwise removal of cheek and neck electrodes, averaged IEDs were downsampled to 219, 204, and 156 EEG channels. Additionally, ESI at the IED’s half-rise was compared to other time points. The respective sources of maximum activity were compared to the resected brain area and postsurgical outcome. Caudal channels had disproportionately more artefacts. In 30 patients with favourable outcome, the 204-channel array yielded the most accurate results with ESI maxima