254 Low Voltage Stereoelectroencephalography (SEEG) Stimulation Suggests Recorded Intracranial Signals and Cortico-Cortical Evoked Potentials (CCEPs) are Often Artefactual Echoes of Each Other

INTRODUCTION: Patients who are candidates for epilepsy surgery often undergo intracranial monitoring with Stereoelectroencephalography (SEEG). Patients occasionally undergo cortico-cortical evoked potentials (CCEP) mapping with SEEG investigations as well, which allows for mapping effective connectivity and functional relationships between different regions of the brain. METHODS: We carried out low voltage stimulation (100 uA-500uA) recordings in nine patients undergoing SEEG for epilepsy surgery. Specifically, two adjacent contacts served as the bipolar stimulation channel, whereas all other implanted contacts served as recording channels. The locations of each contact were determined in three-dimensional patient space, and voltage attenuation models were built and validated. RESULTS: The attenuation in recorded voltages over distances followed a 1/r (unipolar) 1/r2 (bipolar) drop off with a high degree of correlation (R2 > 0.9). Next, the drop-off was theoretically simulated, and a model for the voltage drop-off under a bipolar referencing montage was identified. The expected drop-off in voltage was then used to identify the expected voltage drop-off from the primary CCEP stimulation sites. In addition, the expected drop-off in voltage from the remaining CCEP candidates had their own voltage drop off modeled as well, thus allowing for elimination of all artefactual echoes. The drop-off analysis suggested that across all patients and stimulation trials, up to 80% of previously considered “true” CCEPs, were in fact artefactual echoes of each other and not true CCEPs. Furthermore, around sixpercent of previously “false” CCEPs, were in fact true CCEPs. CONCLUSIONS: A model was built which allows for determining the expected voltage drop off from a stimulation site due to volume conduction alone, which suggests that the number of true-CCEPs in the literature is likely being significantly over-reported.