Quantitative Investigation of Implant Effect of Intracranial EEG Electrodes on Cortico-Cortical Evoked Potentials

Intracranial EEG (iEEG) electrodes allow for direct sampling of neural signals from cortical substructures that provide valuable insight for the treatment of medically-refractory epilepsy and other neurological conditions. Changes in neural behavior (i.e., seizure frequency or motor function) have been documented as the result of the implantation of the intracranial electrodes, termed the implant effect, but the mechanism responsible for these changes is not well understood. Implant revision (i.e., the implantation of new iEEG electrodes to sample additional brain regions) during pre-surgical iEEG monitoring for people with medically refractory epilepsy allows for the comparison of pre- and post-implant iEEG data. In this study, we report the findings from a single patient in whom we quantitatively compared the cortico-cortical evoked potentials (CCEPs) elicited in response to single-pulse electrical stimulation (SPES). We found that the implant effect contributed to an overall decrease in neural connectivity as quantified by the root mean square value of the post-stimulus waveform (p < 0.001) and an increase in evoked spectral power (p < 0.001). We also found that responsive CCEPs were significantly more correlated following implant revision than the non-responsive CCEPs (p < 0.001) with the notable exception of several negatively correlated responsive CCEPs recorded in the same hemisphere as the newly implanted electrodes that retained the canonical CCEP waveform but reversed polarity after implant revision. Our findings showed significant changes in the brain's response to stimulation after the addition of iEEG electrodes, which contributes to improved interpretation of CCEPs recorded during SPES.Clinical Relevance - Exploration of the implant effect of intracranial EEG electrodes provides critical clinical insight for the treatment of epilepsy and other neurological disorders. Quantifying the implant effect caused by the insertion of iEEG electrodes will improve the interpretation of evoked potentials and how they can be utilized to better treat these conditions.