Event Abstract Back to Event The Effect of Spatiotemporal Signal Space Separation (tSSS) on the Localization of Interictal Spikes Michael Funke1* and Samu Taulu2 1 University of Utah, United States 2 Elekta Oy, Finland Introduction: An increasing number of patients with intractable epilepsy are being treated with the vagus nerve stimulator (VNS) while in pursuit of resective neurosurgery. In addition, significant artifact contamination can be caused due to prior craniotomies and dental work. Processing such artifact contaminated clinical data using tSSS seems a promising solution for interference removal. But can localization results obtained from tSSS processed data be trusted? In this study, we investigate the effect of the tSSS on the localization of interictal transients in uncontaminated data sets. Methods: MEG data from ten (10) clinical patients with intractable epilepsy were acquired with a 306-channel whole-head MEG system. Simultaneous EEG was recorded using a 60-channel electrode array. Sleep was induced by prior sleep deprivation. Approximately 60 minutes of continuous data were recorded. None of the patients demonstrated significant artifacts. The spatiotemporal signal space separation method (tSSS) [1], an extended version of the spatial SSS [2], was applied off-line to the raw data. For each patient data were and bandpass filtered 1-70 Hz and ten (10) interictal spikes were localized using all 102 magnetometers and 204 gradiometers. Localization results for a single dipole fit, Goodness of Fit (GOF) as well as the Confidence Volume were compared for raw data as well as the for tSSS processed data. Resuts: The localization results between raw data and tSSS processed data across subjects showed a difference of 1.69 mm (SD: 0.52 mm; range: 1.10 mm - 2.41 mm). The GOF differences across subjects improved on average by 12.59% (SD: 4.82%; range: 5.55% - 18.10%). The Confidence volume decreased by 0.02 mm³ (SD: 0.04 mm³; range: 0 mm³ - 0.08 mm³). Conclusions: Source localization of interictal spikes in tSSS filtered data changed the localization on average by less than 2 mm, compared to unfiltered data, while GOF of the fitted dipoles increased noticeably. The Confidence Volume decreased minimally. Overall, tSSS seems to have very little effect on the localization of interictal spikes but improves the GOF due to noise reduction, ergo increasing slightly the SNR of epileptiform transients.